OneBeacon Insurance Group Ltd. could raise $600 million when its shares are floated on the New York Stock Exchange today.

Bermuda-based White Mountains Insurance Group, Ltd. said last night it had priced the 24 million shares its subsidiary is offering at $25 per share.

The shares will trade under the symbol “OB” beginning on November 9.

White Mountains will receive all of the expected proceeds of the offering.

The underwriters have the option to purchase up to an additional 3.6 million shares from the selling shareholder at the initial public offering price less the underwriting discount. Following the completion of the offering, White Mountains will remain OneBeacon’s majority owner.

Based on OneBeacon’s September 30, 2006 pro forma book value of $1.68 billion, White Mountains expects the transaction to increase White Mountains’ tangible book value per share by $16 per share or $19 per share if the underwriters exercise their over-allotment option in full.