The stolen client information is limited to accounts in Switzerland, excluding ex-HSBC Guyerzeller accounts, the bank added. There is no data compromised for any branches of the bank outside Switzerland, which operate on separate systems and security, or other entities within the HSBC Group, the bank noted.

"We deeply regret this situation and unreservedly apologise to our clients for this threat to their privacy," said Alexandre Zeller, CEO of HSBC Private Bank (Suisse) SA. "We are determined to protect our clients' interests and are taking every necessary measure to do so, actively contacting all our clients with Swiss-based accounts."

Copies of a significant portion of the data were returned to the bank on Mar 3, 2010 by the Swiss Federal Prosecutor, said HSBC, adding that the French authorities had previously seized the files from the former IT employee, who fled to France while under investigation, before passing copies to the Swiss Federal Prosecutor.