GENEVA — Information on 15,000 HSBC customers with Swiss accounts has been stolen, the British bank said Thursday, potentially exposing foreign clients to prosecution by tax authorities in their home countries.

A former IT employee of Swiss subsidiary HSBC Private Bank (Suisse) SA, identified by French authorities as Herve Falciani, stole the information between late 2006 and early 2007, the bank said. The accounts were all opened before October 2006.

“We deeply regret this situation and unreservedly apologize to our clients for this threat to their privacy,” said Alexandre Zeller, chief executive of the Swiss subsidiary.

The bank said it has contacted the affected customers and doesn’t believe the stolen data has or will allow any unauthorized person to access the affected accounts. The stolen information only affects accounts in Switzerland with the exception of its former subsidiary HSBC Guyerzeller Bank, it said.

However, the theft could leave some of those account holders exposed to prosecution by tax authorities.

In recent cases of data theft from banks in Switzerland and Liechtenstein, the information was offered to foreign governments seeking to track down nationals who avoided paying their taxes by hiding money in Swiss accounts.

The French government said last year it had obtained a list of 3,000 French HSBC clients compiled from “numerous sources” including the former HSBC employee – identified by prosecutors in the French city of Aix-en-Provence as Falciani.

France later agreed to return the data to Switzerland.

“The French authorities have informed the Swiss authorities that the data they hold will not be used inappropriately,” HSBC said Thursday without elaborating. It remained unclear whether that means France will not use the data to prosecute tax evaders.

HSBC PLC said offering private banking services for rich customers remains “a core business” of the group, which has about 100,000 private banking clients.

Shares in HSBC were flat at 7.03 pounds (US$10.55) on the London exchange.