Exhibit 99(a)

                                  PRESS RELEASE
                              FOR IMMEDIATE RELEASE

CONTACTS: Carmen Duarte                      Richard Angevine
          OneBeacon Insurance Group          Liberty Mutual Insurance Group
          One Beacon Street                  275 Berkley Street
          Boston, MA 02108                   Boston, MA
          p: 617.725.6598 f: 617.725.7357    p: 617.574.6638 f: 617.574.5637
          cduarte@onebeacon.com              richard.angevine@libertymutual.com

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                      LIBERTY MUTUAL AND ONEBEACON PARTNER
                    ON INDEPENDENT AGENCY INSURANCE BUSINESS
     LIBERTY MUTUAL TO ASSUME ONEBEACON'S BUSINESS & OPERATIONS IN 42 STATES
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(BOSTON, MA - SEPTEMBER 5, 2001) - Liberty Mutual Group and OneBeacon, a wholly
owned subsidiary of White Mountains Insurance Group, Ltd. (NYSE ticker: WTM),
have reached an agreement in principle to enter into a partnership designed to
transfer OneBeacon's property and casualty business and staff and operations in
42 states and the District of Columbia to Liberty Mutual. The transaction is
subject to the parties entering into definitive agreements. Under the proposed
agreement, Liberty Mutual will begin to effect the orderly transition of the
business, operations and staff immediately and will assume all new and renewal
business effective November 1, 2001. OneBeacon will continue to participate in
the future results of this and other Liberty Mutual business for up to five
years through reinsurance agreements. Service agreements will be put in place to
ensure a smooth transition.

OneBeacon's $1.5 billion book of personal and commercial accounts as well as
most of the 2,600 employees in those states will be integrated into Liberty
Mutual's Regional Agency Markets business unit. The portion of OneBeacon's
business being transferred in the Pacific Northwest will be managed by Liberty
Northwest, a separate company in the Liberty Mutual Group.


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ONEBEACON AND LIBERTY MUTUAL PRESS RELEASE (PAGE TWO)

SEPTEMBER 5, 2001

This will give the Liberty Mutual Group one of the largest organizations of
regional insurance operations in the United States solely focused on independent
agents. Liberty Mutual and OneBeacon have agreed on a structure that will
provide continuity for the transferred agents and employees.

OneBeacon is a leading market for independent agents in New England, New York
and New Jersey and this partnership will allow OneBeacon to focus on those
Northeast operations. OneBeacon will also continue to operate in selected
specialty businesses, including Folksamerica Re, International Marine
Underwriters and others. As the partnership does not apply to these markets,
OneBeacon and Liberty Mutual will remain competitors in these regions.

Liberty Mutual has been building a national organization of regional property
and casualty companies that distribute insurance products solely through
independent agents. Those companies currently have in total direct written
premiums of approximately $2.1 billion. They are:

o          Northeast - Peerless Insurance

o          Midwest - Indiana Insurance

o          Mountain - Colorado Casualty Insurance Co.

o          California - Golden Eagle Insurance Corp.

o          Southeast - Montgomery Insurance Companies

o          Specialty - Summit Holdings Southeast and GoAmerica Auto Insurance

In addition, Liberty Northwest has been a specialty workers compensation
provider in the Pacific Northwest since 1984.


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ONEBEACON AND LIBERTY PRESS RELEASE (PAGE THREE)

SEPTEMBER 5, 2001

"This partnership strengthens our position in the independent agency
distribution system," said Edmund F. Kelly, Liberty Mutual Group chairman,
president and chief executive officer. "With this, Liberty Mutual will have
significant presence in the agency market in all regions and, in aggregate one
of the largest regionally based property and casualty books in the United
States."

Roger Jean, president of Liberty Mutual's Regional Agency Markets business unit,
said the transfer enhances the unit's scope of operations. "This agreement
broadens our reach in meeting the needs of independent agents and their clients.
We look forward to working with OneBeacon employees, who are known for their
professionalism and service to the independent agency system."

OneBeacon chairman Jack Byrne said, "We are comfortable that we can offer our
transferring agents and employees a partnership that provides continuity and a
fine future with a responsible financial institution. In Liberty Mutual, we have
found a partner that has figured out how to offer employees, independent agents
and customers the benefits of a regional company, backed by the strength of a
strong national brand."

Ray Barrette, managing director and CEO of OneBeacon added, "We are committed to
improving OneBeacon's position as an independent agency company in the
Northeast. Our renewed focus on our heartland territories of New England and New
York can only enhance our ability to deliver real value to our agency partners."


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ONEBEACON AND LIBERTY PRESS RELEASE (PAGE FOUR)

SEPTEMBER 5, 2001

Boston-based Liberty Mutual is a diversified international group of insurance
companies and one of the largest multi-line insurers in the North American
property and casualty industry. The group has more than $54 billion in
consolidated assets, more than $67 billion in assets under management, $13.5
billion in consolidated revenue and ranks 121st on the Fortune 500 list of
largest corporations in the United States. The A.M. Best Company has rated
Liberty "A+" (Superior). Along with being the leading provider of workers
compensation insurance, programs and services in the United States for 65 years,
Liberty Mutual provides a wide range of products and services, including general
liability, commercial auto and business property; group life and disability;
private passenger auto and homeowners insurance; individual life and annuities,
international programs and financial services.

OneBeacon was introduced as the group name on June 1, 2001, concurrent with
White Mountains Insurance Group's acquisition of the former CGU U.S. property
and casualty operations from CGNU plc. Headquartered in Boston,
Massachusetts, OneBeacon has an A.M. Best rating of A (excellent). OneBeacon
offers a wide range of personal, commercial and specialty products and
services, which are sold exclusively through select independent agents.

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SAFE HARBOR STATEMENT UNDER THE PRIVATE SECURITIES LITIGATION REFORM ACT OF 1995

The information contained in this press release may contain forward looking
statements within the meaning of the "safe harbor" provisions of the United
States Private Securities Litigation Reform Act of 1995. White Mountains
cautions that a number of important factors could cause actual results to differ
materially from the plans, objectives,


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ONEBEACON AND LIBERTY PRESS RELEASE (PAGE FIVE)

SEPTEMBER 5, 2001

expectations, estimates and intentions expressed in forward-looking statements
made by White Mountains. These factors include: (i) competitive forces,
including the conduct of other property and casualty insurers and reinsurers,
(ii) changes in domestic or foreign laws or regulations applicable to White
Mountains, its competitors or its clients, (iii) an economic downturn or other
economic conditions (such as a rising interest rate environment) adversely
affecting White Mountains' financial position, (iv) loss reserves and other
balance sheet items established by White Mountains subsequently proving to have
been inadequate and (v) the failure of White Mountains to enter into or to
complete the proposed transaction with Liberty Mutual under expected terms or at
all. White Mountains cautions that the foregoing list of important factors is
not exhaustive. In any event, such forward-looking statements made by White
Mountains speak only as of the date on which they are made, and White Mountains
does not undertake any obligation to update or revise such statements as a
result of new information, future events or otherwise.


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