UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q

(Mark One)

For the quarterly period ended March 31, 2013

OR

For the transition period fromto

Commission file number 1-32599

WILLIAMS PARTNERS L.P.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (918) 573-2000

NO CHANGE

(Former name, former address and former fiscal year, if changed since last report)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yesþ   No¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files). Yesþ   No¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes¨ Noþ

The registrant had 413,900,699 common units outstanding as of May 7,July 31, 2013.

Williams Partners L.P.

Index

Certain matters contained in this report include “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. These forward-looking statements relate to anticipated financial performance, management’s plans and objectives for future operations, business prospects, outcome of regulatory proceedings, market conditions and other matters.

All statements, other than statements of historical facts, included in this report that address activities, events or developments that we expect, believe or anticipate will exist or may occur in the future are forward-looking statements. Forward-looking statements can be identified by various forms of words such as “anticipates,” “believes,” “seeks,” “could,” “may,” “should,” “continues,” “estimates,” “expects,” “assumes,” “forecasts,” “intends,” “might,” “goals,” “objectives,” “targets,” “planned,” “potential,” “projects,” “scheduled,” “will,” “guidance,” “outlook,” “in service date” or other similar expressions. These statements are based on management’s beliefs and assumptions and on information currently available to management and include, among others, statements regarding:

Forward-looking statements are based on numerous assumptions, uncertainties, and risks that could cause future events or results to be materially different from those stated or implied in this report. Limited partner units are inherently different from the capital stock of a corporation, although many of the business risks to which we are subject are similar to those that would be faced by a corporation engaged in a similar business. You should carefully consider the risk factors discussed below in addition to the other information in this report. If any of the following risks were actually to occur, our business, results of operations, and financial condition could be materially adversely affected. In that case, we might not be able to pay distributions on our common units, the trading price of our common units could decline, and unitholders could lose all or part of their investment. Many of the factors that will determine these results are beyond our ability to control or predict. Specific factors that could cause actual results to differ from results contemplated by the forward-looking statements include, among others, the following:

Given the uncertainties and risk factors that could cause our actual results to differ materially from those contained in any forward-looking statement, we caution investors not to unduly rely on our forward-looking statements. We disclaim any obligations to and do not intend to update the above list or to announce publicly the result of any revisions to any of the forward-looking statements to reflect future events or developments.

In addition to causing our actual results to differ, the factors listed above and referred to below may cause our intentions to change from those statements of intention set forth in this report. Such changes in our intentions may also cause our results to differ. We may change our intentions, at any time and without notice, based upon changes in such factors, our assumptions, or otherwise.

Because forward-looking statements involve risks and uncertainties, we caution that there are important factors, in addition to those listed above, that may cause actual results to differ materially from those contained in the forward-looking statements. For a detailed discussion of those factors, see Part I, Item 1A. Risk Factors in our Annual Report on Form10-K for the year ended December 31, 2012.2012, and Part II, Item 1A. Risk Factors of this Form 10-Q.

PART I – FINANCIAL INFORMATION

Williams Partners L.P.

Consolidated Statement of Comprehensive Income

(Unaudited)

See accompanying notes.

Williams Partners L.P.

Consolidated Balance Sheet

(Unaudited)

See accompanying notes.

Williams Partners L.P.

Consolidated Statement of Changes in Equity

(Unaudited)

See accompanying notes.

Williams Partners L.P.

Consolidated Statement of Cash Flows

(Unaudited)

See accompanying notes.

Williams Partners L.P.

Notes to Consolidated Financial Statements

(Unaudited)

Note 1. General and Basis of Presentation

General

Our accompanying interim consolidated financial statements do not include all the notes in our annual financial statements and, therefore, should be read in conjunction with the consolidated financial statements and notes thereto for the year ended December 31, 2012 in Exhibit 99.1 of our Form 8-K dated May 13, 2013 (2012 Annual Report on Form 10-K.Financial Statements). The accompanying unaudited financial statements include all normal recurring adjustments that, in the opinion of management, are necessary to present fairly our interim financial statements.

The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates.

Unless the context clearly indicates otherwise, references in this report to “we,” “our,” “us,” or similar language refer to Williams Partners L.P. and its subsidiaries.

We are a publicly traded Delaware limited partnership. Williams Partners GP LLC, a Delaware limited liability company wholly owned by The Williams Companies, Inc. (Williams), serves as our general partner. As of March 31,June 30, 2013, Williams owns an approximate 66 percent limited partner interest, a 2 percent general partner interest and incentive distribution rights (IDRs) in us. All of our activities are conducted through Williams Partners Operating LLC, an operating limited liability company (wholly owned by us).

Basis of Presentation

Organizational restructuring

Following Williams’ spin-off of WPX Energy, Inc. (WPX) at the end of 2011 and in consideration of the growth plans of the ongoing business, Williams initiated an organizational restructuring evaluation to better align resources to support an overall business strategy to provide large-scale energy infrastructure designed to maximize the opportunities created by the vast supply of natural gas, natural gas products, and crude oil that exists in North America. As a result of this review, a new structure was implemented effective January 1, 2013, that generally organizes our businesses into geographically based operating areas. We have changed our segment reporting structure to align with the new operating areas resulting from the organizational restructuring, as this is consistent with the manner in which our Chief Operating Decision Maker evaluates performance and makes resource allocation decisions. Beginning in the first quarter of 2013, our reportable segments are Northeast G&P, Atlantic-Gulf, West, and NGL & Petchem Services.

Northeast G&P is comprised of our midstream gathering and processing businesses in the Marcellus and Utica shale regions, as well as a 51 percent equity investment in Laurel Mountain Midstream, LLC (Laurel Mountain) and a 47.5 percent equity investment in Caiman Energy II, LLC (Caiman).

Atlantic-Gulf is comprised of our interstate natural gas pipeline, Transcontinental Gas Pipe Line Company, LLC (Transco), and significant natural gas gathering and processing and crude production handling and transportation in the Gulf Coast region, as well as a 50 percent equity investment in Gulfstream Natural Gas System, L.L.C. (Gulfstream), a 5141 percent consolidated interest in Constitution Pipeline Company, LLC (Constitution), and a 60 percent equity investment in Discovery Producer Services LLC (Discovery).

West is comprised of our gathering, processing and treating operations in New Mexico, Colorado, and Wyoming and our interstate natural gas pipeline, Northwest Pipeline GP (which was changed to Northwest Pipeline LLC on July 1, 2013) (Northwest Pipeline).

Notes (Continued)

NGL & Petchem Services is comprised of our natural gas liquid (NGL) and natural gas marketing business, an NGL fractionator and storage facilities near Conway, Kansas, a 50 percent equity investment in Overland Pass Pipeline, LLC (OPPL), and an 83.3 percent undivided interest in an olefins production facility in Geismar, Louisiana, along with a refinery grade propylene splitter and pipelines in the Gulf Coast region.

Other

As disclosed in our 2012 Annual Report on Form 10-K, in November 2012Financial Statements, we acquired an entity in November 2012 that holds an 83.3 percent undivided interest in an olefins-production facility in Geismar, Louisiana and associated assets from Williams. As a result, prior period financial statement amounts and disclosures have been recast for this transaction. The effect of recasting our financial statements to account for this transaction increased net income $60 million for the three months ended March 31, 2012. This acquisition does not impact historical earnings per unit as pre-acquisition earnings were allocated to our general partner. In March 2013, we received $25 million in cash from Williams and Williams waived $4 million in payments on its IDRs with respect to our next quarterly distribution related to a working capital adjustment associated with the acquisition.

The entity acquired in the Geismar Acquisition was an affiliate of Williams at the time of the acquisition; therefore, the acquisition was accounted for as a common control transaction, similar to a pooling of interests, whereby the assets and liabilities of the acquired entity waswere combined with ours at their historical amounts. As a result, prior period financial statement amounts and disclosures have been recast for this transaction. The effect of recasting our financial statements to account for this transaction increased net income $50 million and $110 million for the three and six months ended June 30, 2012, respectively. This acquisition does not impact historical earnings per common unit as pre-acquisition earnings were allocated to our general partner. In March 2013, we received $25 million in cash from Williams and Williams waived $4 million in payments on its historical amount.IDRs with respect to our May 2013 distribution related to a working capital adjustment associated with the acquisition.

Also as disclosed in our 2012 Annual Report on Form 10-K,Financial Statements, we have revised the overall presentation of our Consolidated Statement of Comprehensive Income, including the separate presentation ofservice revenues,product sales,product costs, anddepreciation and amortization expenses. All prior periods presented have been recast, along with corresponding information presented in the Notes to Consolidated Financial Statements, to reflect this change.

Note 2. Variable Interest Entities

Consolidated VIEs

We consolidate the activities of variable interest entities (VIEs) of which we are the primary beneficiary. The primary beneficiary of a VIE is the entity that has both (1) the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and (2) the obligation to absorb losses or the right to receive benefits that could be significant to the VIE. As of March 31,June 30, 2013, we have the following consolidated VIEs:

Notes (Continued)

Notes (Continued)

The following table presents amounts included in our Consolidated Balance Sheet that are for the use or obligation of these VIEs:VIEs, which are joint projects in the development and construction phase:

Nonconsolidated VIEs

We have also identified certain interests in VIEs where we are not the primary beneficiary. These include our equity method investmentsinclude:

Notes (Continued)

Note 3. Allocation of Net Income and Distributions

The allocation of net income between our general partner and limited partners is as follows:

Notes (Continued)

We paid or have authorized payment of the following partnership cash distributions during 2012 and 2013 (in millions, except for per unit amounts):

Notes (Continued)

The 2012 and 2013 cash distributions paid to our general partner in the table above have been reduced by a total of $79$105 million resulting from the temporary waiver of IDRs associated with certain assets acquired in 2012.

Note 4. Other Accruals

On June 13, 2013, an explosion and fire occurred at our Geismar olefins plant located south of Baton Rouge, in a remote industrial complex, that resulted in the tragic deaths of two affiliate employees and injuries of additional affiliate employees and contractors. The fire was extinguished on the day of the incident. The incident (Geismar Incident) rendered the facility temporarily inoperable and resulted in significant human, financial and operational effects.

We have substantial insurance coverage for repair and replacement costs, lost production and additional expenses related to the incident as follows:

We are in the early stages of determining the full extent of property damage and developing claims information for business interruption coverage. Through June 30, 2013, we have expensed $6 million of insurance deductibles inoperating and maintenance expenses in the Consolidated Statement of Comprehensive Income, based on our initial evaluation. Recoveries under our business interruption policy will be recognized upon resolution of any contingencies with the insurer associated with the claim. Such recoveries, when recognized, will be recorded as a gain toother (income) expense – net withincosts and expenses in our Consolidated Statement of Comprehensive Income.

During the second quarter of 2012, we incurred acquisition transaction costs of $16 million related to the acquisition of 100 percent of the ownership interests in Caiman Eastern Midstream, LLC. These costs are included inselling, general, and administrative expenses.

Other (income) expense – net withincosts and expenses for the three and six months ended June 30, 2013 includes a $6 million expense related to the portion of the Eminence abandonment regulatory asset that will not be recovered through rates, pursuant to Transco’s agreement in principle associated with its general rate case filing. (See Note 9.) We also recognized income of $12 million related to insurance recoveries associated with this event that we consider probable of collection. Additionally, we recorded charges of $2 million during the three and six months ended June 30, 2013 and $9 million and $15 million during the three and six months ended June 30, 2012, respectively, related to project development costs associated with natural gas pipeline expansion projects.

Other income (expense) – net belowoperating income for the three and six months ended June 30, 2013, includes a charge of $14 million associated with the impact of a second quarter Texas franchise tax law change.

Notes (Continued)

Note 5. Inventories

Note 5.6. Debt and Banking Arrangements

Credit Facility

Letter of credit capacity under our $2.4 billion credit facility is $1.3 billion. At March 31,June 30, 2013, no letters of credit have been issued and $250 million ofno loans are outstanding under our credit facility.

On July 31, 2013, we amended our $2.4 billion credit facility to increase the aggregate commitments to $2.5 billion and extend the maturity date to July 31, 2018. Additionally, Transco and Northwest Pipeline are each able to borrow up to $500 million under the amended credit facility to the extent not otherwise utilized by the other co-borrowers. Our credit facility may also, under certain conditions, be increased up to an additional $500 million.

Commercial Paper Program

In March 2013, we initiated a commercial paper program. The program allows a maximum outstanding amount at any time of $2 billion of unsecured commercial paper notes. The maturities of the commercial paper notes will vary but may not exceed 397 days from the date of issuance. The commercial paper notes will beare sold under customary terms in the commercial paper market and will beare issued at a discount from par, or, alternatively, will beare sold at par and bear varying interest rates on a fixed or floating basis. Proceeds from these notes are expected to be used to fund plannedfor general partnership purposes, including funding capital expenditures, working capital, and for other general partnership purposes. We have not yet issued any notes under thisdistributions. At June 30, 2013, $710 million of commercial paper program.is outstanding at a weighted average interest rate of 0.42 percent.

Note 6.7. Partners’ Capital

Notes (Continued)

In March 2013, we completed an equity issuance of 14,250,000 common units, including 3,000,000 common units sold to Williams in a private placement. Subsequently, the underwriters exercised their option to purchase 1,687,500 common units. The net proceeds of approximately $760 million were used to repay amounts outstanding under our revolver.

Note 7.8. Fair Value Measurements

The following table presents, by level within the fair value hierarchy, certain of our financial assets and liabilities. The carrying values of cash and cash equivalents, accounts receivable, commercial paper, and accounts payable approximate fair value because of the short-term nature of these instruments. Therefore, these assets and liabilities are not presented in the following table.

Notes (Continued)

Fair Value Methods

We use the following methods and assumptions in estimating the fair value of our financial instruments:

Assets and liabilities measured at fair value on a recurring basis

ARO Trust investments: Transco deposits a portion of its collected rates, pursuant to its 2008 rate case settlement, into an external trust (ARO Trust) that is specifically designated to fund future asset retirement

Notes (Continued)

obligations. The ARO Trust invests in a portfolio of actively traded mutual funds that are measured at fair value on a recurring basis based on quoted prices in an active market, is classified as available-for-sale, and is reported inregulatory assets, deferred charges, and other in the Consolidated Balance Sheet. Both realized and unrealized gains and losses are ultimately recorded as regulatory assets or liabilities.

Energy derivatives: Energy derivatives include commodity based exchange-traded contracts and over-the-counter (OTC) contracts, which consist of physical forwards, futures, and swaps that are measured at fair value on a recurring basis. The fair value amounts are presented on a gross basis and do not reflect the netting of asset and liability positions permitted under the terms of our master netting arrangements. Further, the amounts do not include cash held on deposit in margin accounts that we have received or remitted to collateralize certain derivative positions. Energy derivatives assets are reported inother current assetsandregulatory assets, deferred charges, and otherin the Consolidated Balance Sheet. Energy derivatives liabilities are reported inother accrued liabilities andregulatory liabilities, deferred income, and otherin the Consolidated Balance Sheet.

Reclassifications of fair value between Level 1, Level 2, and Level 3 of the fair value hierarchy, if applicable, are made at the end of each quarter. No transfers between Level 1 and Level 2 occurred during the threesix months ended March 31,June 30, 2013 or 2012.

Additional fair value disclosures

Notes receivable and other: The disclosed fair value of our notes receivable is primarily determined by an income approach which considers the underlying contract amounts and our assessment of our ability to recover these

Notes (Continued)

amounts. The current portion is reported intrade accounts and notes receivable, and the noncurrent portion is reported inregulatory assets, deferred charges, and other in the Consolidated Balance Sheet.

Long-term debt: The disclosed fair value of our long-term debt is determined by a market approach using broker quoted indicative period-end bond prices. The quoted prices are based on observable transactions in less active markets for our debt or similar instruments.

Guarantees

We are required by our revolving credit agreement to indemnify lenders for certain taxes required to be withheld from payments due to the lenders and for certain tax payments made by the lenders. The maximum potential amount of future payments under these indemnifications is based on the related borrowings and such future payments cannot currently be determined. These indemnifications generally continue indefinitely unless limited by the underlying tax regulations and have no carrying value. We have never been called upon to perform under these indemnifications and have no current expectation of a future claim.

Note 8.9. Contingent Liabilities

Environmental Matters

We are a participant in certain environmental activities in various stages including assessment studies, cleanup operations and remedial processes at certain sites, some of which we currently do not own. We are monitoring these sites in a coordinated effort with other potentially responsible parties, the U.S. Environmental Protection Agency (EPA), and other governmental authorities. We are jointly and severally liable along with unrelated third parties in some of these activities and solely responsible in others. Certain of our subsidiaries have been identified as potentially responsible parties at various Superfund and state waste sites. In addition, these subsidiaries have incurred, or are alleged to have incurred, various other hazardous materials removal or remediation obligations under environmental laws. As of March 31,June 30, 2013, we have accrued liabilities totaling $20 million for these matters, as discussed below. Our accrual reflects the most likely costs of cleanup, which are generally based on completed assessment studies, preliminary results of studies or our experience with other similar cleanup operations. Certain

Notes (Continued)

assessment studies are still in process for which the ultimate outcome may yield significantly different estimates of most likely costs. Any incremental amount in excess of amounts currently accrued cannot be reasonably estimated at this time due to uncertainty about the actual number of contaminated sites ultimately identified, the actual amount and extent of contamination discovered and the final cleanup standards mandated by the EPA and other governmental authorities.

The EPA and various state regulatory agencies routinely promulgate and propose new rules, and issue updated guidance to existing rules. More recent rules and rulemakings include, but are not limited to, rules for reciprocating internal combustion engine maximum achievable control technology, new air quality standards for ground level ozone, and one hour nitrogen dioxide emission limits. We are unable to estimate the costs of asset additions or modifications necessary to comply with these new regulations due to uncertainty created by the various legal challenges to these regulations and the need for further specific regulatory guidance.

Our interstate gas pipelines are involved in remediation activities related to certain facilities and locations for polychlorinated biphenyls, mercury, and other hazardous substances. These activities have involved the EPA and various state environmental authorities, resulting in our identification as a potentially responsible party at various Superfund waste sites. At March 31,June 30, 2013, we have accrued liabilities of $9$11 million for these costs. We expect that these costs will be recoverable through rates.

We also accrue environmental remediation costs for natural gas underground storage facilities, primarily related to soil and groundwater contamination. At March 31,June 30, 2013, we have accrued liabilities totaling $11$9 million for these costs.

Notes (Continued)

Geismar Incident

As a result of the previously discussed Geismar Incident, there were two fatalities and numerous individuals (including affiliate employees and contractors) reported injuries, which varied from minor to serious. We are cooperating with the Occupational Safety and Health Administration, the Chemical Safety Board, and the EPA to conduct investigations to determine the cause of the incident. Also, on June 28, 2013, the Louisiana Department of Environmental Quality issued a Consolidated Compliance Order & Notice of Potential Penalty to Williams Olefins, L.L.C. that consolidates claims of unpermitted emissions and other deviations under the Clean Air Act that the parties had been negotiating since 2010 and alleged unpermitted emissions arising from the Geismar Incident. Any potential fines and penalties from these agencies would not be covered by our insurance policy. Additionally, multiple lawsuits, including class actions for alleged offsite impacts, property damage, and personal injury, have been filed against various of our subsidiaries.

Due to the recent nature of the incident, the preliminary and ongoing investigation into its cause, and the limited information available associated with the filed lawsuits, which do not specify any amounts for claimed damages, we cannot reasonably estimate a range of potential loss related to these lawsuits at this time.

Rate Matters

On August 31, 2012, Transco submitted to the Federal Energy Regulatory Commission (FERC)FERC a general rate filing principally designed to recover increased costs and to comply with the terms of the settlement in its prior rate proceeding. The new rates became effective March 1, 2013, subject to refund and the outcome of a hearing. We have reached an agreement in principle with the participants that would resolve all issues in this proceeding without the need for a hearing. Final resolution of the rate case is subject to the filing of a formal stipulation and agreement and subsequent approval by the FERC. We have provided a reserve for rate refunds which we believe is adequate for any refunds that may be required.

On August 31, 2006, Transco submitted to the FERC a general rate filing principally designed to recover increased costs. The rates became effective March 1, 2007, subject to refund and the outcome of a hearing. All issues in this proceeding except one have been resolved by settlement.

The one issue reserved for litigation or further settlement relates to Transco’s proposal to change the design of the rates for service under one of its storage rate schedules, which was implemented subject to refund on March 1, 2007. A hearing on that issue was held before a FERC Administrative Law Judge (ALJ) in July 2008. In November 2008, the ALJ issued an initial decision in which he determined that Transco’s proposed incremental rate design is unjust and unreasonable. On January 21, 2010, the FERC reversed the ALJ’s initial decision, and approved our proposed incremental rate design. Certain parties sought rehearing of the FERC’s order and, on April 2, 2012, the FERC denied the rehearing request. On June 1, 2012, one party filed an appeal in the U.S. Court of Appeals for the D.C. Circuit challenging the FERC’s orders approving our rate design proposal.

Other

In addition to the foregoing, various other proceedings are pending against us which are incidental to our operations.

Summary

We estimate that for all matters for which we are able to reasonably estimate a range of loss, including those noted above and others that are not individually significant, our aggregate reasonably possible losses beyond amounts accrued for all of our contingent liabilities are immaterial to our expected future annual results of

Notes (Continued)

operations, liquidity, and financial position. These calculations have been made without consideration of any potential recovery from third parties. We disclose all significant matters for which we are unable to reasonably

Notes (Continued)

estimate a range of possible loss.

Note 9.10. Segment disclosuresDisclosures

Our reportable segments are Northeast G&P, Atlantic-Gulf, West, and NGL & Petchem Services. (See Note 1.)

Performance Measurement

We currently evaluate segment operating performance based onsegment profit (loss)from operations, which includessegment revenuesfrom external and internal customers, segment costs and expenses,and equity earnings (losses).General corporate expenses representselling, general, and administrative expenses that are not allocated to our segments. Intersegment revenues primarily represent the sale of NGLs from our natural gas processing plants to our marketing business and are generally accounted for at current market prices as if the sales were to unaffiliated third parties.

The following table reflects the reconciliation ofsegment revenues andsegment profit (loss)torevenues andoperating incomeas reported in the Consolidated Statement of Comprehensive Income.