HSBC has said approximately 15,000 accounts at its Swiss private banking unit were compromised after an employee allegedly stole data, some of which ended up in the hands of French tax authorities.

The latest figure is one thousand five hundred times higher than the one the bank gave in December, when HSBC said the number of account records taken was less than 10. HSBC said it does not think the records could be used to access an account.

The data was allegedly stolen by a former IT employee about three years ago, HSBC said. The employee left Switzerland, and French authorities ended up with the files, which were then passed to the Swiss Federal Prosecutor. French authorities had been investigating up to 3,000 people thought to be avoiding taxes.

In December, CIO 100 listed HSBC said the former employee had been charged by Swiss prosecutors after it filed a criminal complaint in 2008. Swiss prosecutors are continuing their investigation, HSBC said.

The data involved clients who had accounts with HSBC's Private Bank in Switzerland before October 2006. HSBC received copies of the data from the Swiss Federal Prosecutor on March 3.

HSBC Private Bank has spent 100 million Swiss francs (£62 million) to upgrade security systems, it said.