UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934

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¨  Preliminary Proxy Statement

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Ambac Financial Group, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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May 23, 2008 Dear Fellow Shareholder: We have previously sent to you proxy material for the Annual Meeting of Shareholders of Ambac Financial Group, Inc., to be held on Tuesday, June 3, 2008. Your Board of Directors unanimously recommends that shareholders vote FOR Proposals 1, 2, 3, 4 and 5. Your vote is important, no matter how many or how few shares you may own. Whether or not you have already done so, please vote TODAY by telephone, via the Internet, or by signing, dating and returning the enclosed proxy card or voting instruction form in the envelope provided. Very truly yours, Michael A. Callen Chairman and Interim President and Chief Executive Officer

IMPORTANT – TIME IS SHORT! You can vote your shares by telephone or via the Internet. Please follow the easy instructions on the enclosed proxy card or voting instruction form. If you have any questions, or need assistance in voting your shares, please call our proxy solicitor: BNY MELLON SHAREOWNER SERVICES TOLL FREE, at 1-888-867-6202