Exhibit 99.(a)(1)(D)

                      WHITE MOUNTAINS INSURANCE GROUP, LTD.
                           OFFER TO PURCHASE FOR CASH
                      UP TO 1,500,000 OF ITS COMMON SHARES
                            AT $165.00 NET PER SHARE


THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE
AT 12:00 MIDNIGHT, NEW YORK CITY TIME, ON MONDAY, SEPTEMBER 18, 2000,
UNLESS THE OFFER IS EXTENDED.

                                                                 August 21, 2000

To Brokers, Dealers, Commercial Banks,
         Trust Companies and Other Nominees:

         We are enclosing herewith the material listed below relating to the
offer by White Mountains Insurance Group, Ltd., a company organized under the
laws of Bermuda (the "Company"), to purchase up to 1,500,000 of its outstanding
Common Shares, par value $1.00 per share (the "Shares"), for cash at $165.00 per
Share net to the seller, upon the terms and subject to the conditions set forth
in the Offer to Purchase dated August 21, 2000, and in the related Letter of
Transmittal (which together constitute the "Offer"). The Company may elect, but
is not obligated, to purchase additional Shares pursuant to the Offer. THE OFFER
IS NOT CONDITIONED UPON ANY MINIMUM NUMBER OF SHARES BEING VALIDLY TENDERED.

         We have been engaged by the Company as Information Agent with respect
to the Offer. We are asking you to contact your clients for whom you hold Shares
registered in your name (or in the name of your nominee) or who hold Shares
registered in their own names. Please bring the Offer to their attention as
promptly as possible. No fees or commissions (other than fees to the Information
Agent and the Depositary as described in the Offer) will be payable to brokers,
dealers or other persons for soliciting tenders of Shares pursuant to the Offer.
The Company will, however, upon request, reimburse you for customary mailing and
handling expenses incurred by you in forwarding any of the enclosed materials to
your clients. No shareholders will be required to pay transfer taxes on the
transfer to the Company of Shares purchased pursuant to the Offer, subject to
Instruction 6 of the Letter of Transmittal.

         For your information and for forwarding to your clients, we are
enclosing the following documents:

         (1)      Offer to Purchase dated August 21, 2000;

         (2)      Specimen Letter of Transmittal to be used by holders of Shares
                  to tender Shares and for the information of your clients;

         (3)      Letter from the Chairman of the Company, John J. Byrne, to
                  shareholders.

         (4)      Form of Notice of Guaranteed Delivery;

         (5)      Guidelines for Certification of Taxpayer Identification Number
                  on Substitute Form W-9;

         (6)      Letter which may be sent to your clients for whose accounts
                  you hold Shares registered in your name (or in the name of
                  your nominee), with space provided for obtaining such clients'
                  instructions with regard to the Offer; and

         (7)      Return envelope addressed to Equiserve Trust Company, the
                  Depositary.

         WE URGE YOU TO CONTACT YOUR CLIENTS PROMPTLY. PLEASE NOTE THAT THE
OFFER, THE PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT,
NEW YORK CITY TIME, ON MONDAY, SEPTEMBER 18, 2000, UNLESS EXTENDED.



         Your communications to shareholders with respect to the Offer will
constitute your representation to the Company that (i) in connection with such
communications you have complied with the applicable requirements of the
Securities Exchange Act of 1934, as amended, and the applicable rules and
regulations thereunder; (ii) if a foreign broker or dealer, you have conformed
to the Rules of Fair Practice of the National Association of Securities Dealers,
Inc. in making such communications; and (iii) in connection with such
communications you have not used any offering materials other than those
furnished by the Company.

         The Offer is not being made to (nor will tenders be accepted from or on
behalf of) holders of Shares residing in any jurisdiction in which the making of
the Offer or the acceptance thereof would not be in compliance with securities
or Blue Sky laws of such jurisdiction.

         Additional copies of the enclosed material may be obtained from the
undersigned. Any questions you may have with respect to the Offer should be
directed to the undersigned at 1-888-756-1854.

                                                        Very truly yours,


                                                        Equiserve Trust Company



NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU OR
ANY OTHER PERSON THE AGENT OF THE COMPANY, THE DEPOSITARY OR THE INFORMATION
AGENT OR AUTHORIZE YOU OR ANY OTHER PERSON TO MAKE ANY STATEMENTS OR USE ANY
MATERIAL ON BEHALF OF ANY OF THEM WITH RESPECT TO THE OFFER, OTHER THAN THE
MATERIAL ENCLOSED HEREWITH AND THE STATEMENTS SPECIFICALLY CONTAINED IN SUCH
MATERIAL.