HSBC announced Thursday the data of 15,000 global clients with accounts with its Swiss private-banking arm HSBC Holding PLC has been stolen.

The stolen data involves accounts oppened at HSBC in Switzerland before October 2006. A former employee of the bank has been accused for carrying out the theft.

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

HSBC said it does not believe that the stolen data has or will allow any third party to access any client account but it has taken action by contacting customers to explain and apologize for the incident.

On March 3, Swiss authorities passed data files to HSBC which showed that client information had been compromised, the bank said in a statement. A former information technology employee at HSBC Holding PLC, reportedly stole the information when he was migrating data. Although the bank has not disclosed his name, his identity has been reported as Herve Falciani.

Falciani allegedly transferred the data from an HSBC computer to a personal device and then tried to sell it to one or more banks in Lebanon, the Wall Street Journal reported.

HSBC admitted in December that account data from clients in Switzerland had been stolen but it said they were fewer than 10 names.

The bank said Thursday it spent more than the equivalent to $93.3 million on upgrading its security since the theft.

The theft represents a major breach f trust for HSBC and at the same time, could expose some of the clients to prosecution by tax authorities.