HSBC has admitted that an IT employee at its Swiss private banking arm stole the account details of some 15,000 customers and passed them on to French tax authorities.
The scale of the breach is a serious embarrassment to HSBC, which initially claimed that no more than ten accounts had been affected when news of the theft first start circulating late last year.
In a statement, HSBC's Swiss private bank now admits that the data relating to 15,000 client accounts was stolen by a former information technology employee about three years ago. On top of that an additional 9000 accounts that had been closed in the past were also affected.
"We deeply regret this situation and unreservedly apologisae to our clients for this threat to their privacy," Alexandre Zeller, chief executive of the Swiss unit, told reporters.
He says the bank has received reassurances from French authorities that the information will not be used "inappropriately".
"The bank does not believe that the stolen data has or will allow any third party to access any client account," the HSBC statement said.
The willingness of foreign tax authorities to pay for information relating to Swiss private bank accounts has been a growing source of diplomatic tension. The Swiss government in January said it plans to draft a new law that would ban officials from co-operating with foreign countries where private account details have been stolen.