An ex-employee of HSBC stole the details of 24,000 clients from a branch in Switzerland, in a hugely embarrassing theft of data for which the bank apologised unreservedly yesterday.

Europe's biggest bank revealed that Hervé Falciani, a former computer specialist who fled to France, had stolen the details from its Geneva private bank three years ago. Some 15,000 of the accounts were still active, HSBC said. The rest were closed.

Finma, the Swiss financial markets regulator, has launched an investigation into the theft. It will examine how so much data could have been stolen and whether HSBC's response had met legal requirements.

Chris Meares, HSBC's head of global private banking, said that such investigations were “normal procedure” and that the bank was working closely with the regulator.

The theft will heighten concerns about client confidentiality in Switzerland and Liechtenstein, which have strict bank secrecy. HSBC said that the stolen information was limited to accounts in Switzerland. “We deeply regret the situation and unreservedly apologise to our clients for this threat to their privacy”, said Alexendre Zeller, head of HSBC's Swiss private bank.

The bank had originally played down the theft, revealed last December, saying that fewer than 10 clients were involved. Mr Zeller said that the statement was based on information from the Swiss Federal Prosecutor, which acted after a tip-off from a Lebanese bank.