A Swiss unit of global banking giant HSBC revealed Thursday that details on 24,000 bank customers may have been leaked in a major security breach that has helped fuel pressure on Switzerland over .
Alexandre Zeller, chief executive of HSBC Private Bank (Switzerland), said 9,000 of those customers had left the bank since the the data theft by a computer expert three years ago, and 15,000 of its remaining customers were concerned by the case.
Switzerland's financial and banking regulator announced that it had launched "formalagainst HSBC" over the security breach, noting that the stolen data was "extensive."
Shortly after the case first came to light in France in December, the bank said that to its knowledge the stolen data concerned fewer than 10 clients.
On Thursday, HSBC Private Bank apologized to its clients in a statement and said it was only just coming to understand the full extent of the leak.
"It is now clear that the theft, which was carried out by an employee of the IT department about three years ago, could concern about 15,000 current clients whose accounts were opened in Switzerland before October 2006," Zeller said.
HSBC insisted that client data in the bank's branches outside Switzerland or other parts of the banking group were not affected because of distinct computer and security systems.
Switzerland is in the processs of striking a string of deals to fall in line with international standards against tax evasion, following months of pressure by the United States and leading European economies.