A.M. Best Co. has affirmed the financial strength rating (FSR) of A (Excellent) and the issuer credit ratings (ICR) of “a” of the operating companies comprising OneBeacon Insurance Group (OneBeacon) (Minnetonka, MN).

Additionally, A.M. Best has affirmed the ICR of “bbb” of OneBeacon’s publicly traded parent, OneBeacon Insurance Group, Ltd. (OneBeacon Ltd.) (Hamilton, Bermuda) [NYSE: OB] and its intermediate holding company, OneBeacon U.S. Holdings, Inc. (OneBeacon U.S.) (headquartered in Minnetonka, MN). Concurrently, A.M. Best has affirmed the ICR of “bbb” and debt ratings of the group’s ultimate parent, White Mountains Insurance Group, Ltd. (White Mountains) (Hamilton, Bermuda) [NYSE: WTM].

At the same time, A.M. Best has affirmed the FSR of B++ (Good) and ICR of “bbb+” of Potomac Insurance Company (Philadelphia, PA), which currently is in run off. The outlook for all the above ratings is stable.

The affirmation of OneBeacon’s ratings reflects its strong risk-adjusted capitalization and solid earnings through its solid underwriting and operating performance. The ratings also consider the financial flexibility of OneBeacon U.S. and OneBeacon Ltd. These positive rating factors are partially offset by OneBeacon’s historical adverse development on its run-off operations.

The affirmation of White Mountains’ ratings acknowledges its strong financial flexibility and the solid operating results generated through its operating subsidiaries.

The FSR of A (Excellent) and ICR of “a” have been affirmed for OneBeacon Insurance Group and its following members:

• Atlantic Specialty Insurance Company

• AutoOne Insurance Company

• AutoOne Select Insurance Company

• The Camden Fire Insurance Association

• The Employers’ Fire Insurance Company

• Essentia Insurance Company

• Homeland Insurance Company of New York

• The Northern Assurance Company of America

• OneBeacon America Insurance Company

• OneBeacon Insurance Company

• OneBeacon Midwest Insurance Company

• Pennsylvania General Insurance Company

• Traders & General Insurance Company
      The following debt rating has been affirmed:

OneBeacon U.S. Holdings, Inc.—
-- “bbb” on $270 million 5.875% senior unsecured notes, due 2013
The following indicative ratings under the shelf registration have been affirmed:

OneBeacon U.S. Holdings, Inc. and OneBeacon Insurance Group, Ltd.—
-- “bbb” on senior unsecured debt
-- “bbb-“ on subordinate debt
-- “bb+” on preferred stock

OneBeacon U.S. Holdings Trust I, II, III—
-- “bb+” on preferred stock

The principal methodology used in determining these ratings is Best’s Credit Rating Methodology -- Global Life and Non-Life Insurance Edition , which provides a comprehensive explanation of A.M. Best’s rating process and highlights the different rating criteria employed. Additional key criteria utilized include: “Risk Management and the Rating Process for Insurance Companies”; “Understanding BCAR for Property/Casualty Insurers”; “Ratings Members of Insurance Groups”; “The Treatment of Terrorism Risk in the Rating Evaluation”; “A.M. Best’s Ratings & the Treatment of Debt”; “Understanding Universal BCAR”; and “Natural Catastrophe Stress Test Methodology.” Methodologies can be found at www.ambest.com/ratings/methodology.

Founded in 1899, A.M. Best Company is the world’s oldest and most authoritative insurance rating and information source. For more information, visit www.ambest.com.

Copyright © 2011 by A.M. Best Company, Inc. ALL RIGHTS RESERVED.

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