stake in bond insurer Financial Security Assurance to Belgo-French banking group Dexia Credit for $620.4 million.

Earlier, Dexia said it would acquire the whole of Financial Security for $76 per share, or about $2.6 billion.

White Mountains said it had agreed to sell its subsidiary White Mountains Holdings, the largest shareholder of Financial Security with about 26 percent of its diluted shares, to Dexia. White Mountains said the deal would increase its book value at December 31, 1999, by $270.3 million, or $45.47 per share.