The bank said that it did not believe the data will allow anyone to access the accounts.

However 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.

A former IT employee of Swiss subsidiary HSBC Private Bank (Suisse) SA, Herve Falciani, stole the information between late 2006 and early 2007, the bank said.

The accounts, held by individuals worldwide, were all opened before October 2006 and around 9,000 have since been closed.

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

The stolen information only affects accounts in Switzerland with the exception of its former subsidiary HSBC Guyerzeller Bank, it said.

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 files to Switzerland, who in turn handed "copies of a significant portion of the data" back to the bank on March 3, HSBC said. "Based on the facts it would appear that the French authorities have copies and the Swiss authorities have copies," an HSBC spokesman said.

The bank said the French had informed their Swiss counterparts that the data they still hold would "not be used inappropriately." It remained unclear whether that means France will not use it to prosecute tax evaders.