Group Inc., headed by longtime insurance executive John J. Byrne, said yesterday it will buy the US property-casualty insurance operations of British insurer CGNU PLC for $2.1 billion.

Byrne previously ran the auto insurer Geico Inc., now a unit of Warren Buffett's Berkshire Hathaway Inc. White Mountains said Berkshire Hathaway will invest up to $300 million in the company as part of the deal.

News of the purchase, which also includes the assumption of $500 million of debt, sent shares of White Mountains surging 14.6 percent, or $25.50, to $200 in midday trading on the New York Stock Exchange.

White Mountains, based in Hanover, New Hampshire, owns Folksamerica, a reinsurance concern, and a 50 percent stake in Main Street America Holdings, a property-casualty insurance company jointly owned with National Grange Mutual Insurance.

The Boston-based business being purchased by White Mountains includes commercial and personal lines of insurance.

CGNU put the US property-casualty business up for sale earlier this year, saying it wanted to withdraw from the competitive US market.