HSBC says Swiss data theft affects 24,000 accounts

HSBC shares lost further ground after the statement, off 1.2 percent at 697.8 pence at 1 p.m. British time, underperforming a 0.5 percent weaker European banking sector <.SX7P>.

HSBC said in December that an ex-employee had stolen data from its Swiss private bank's headquarters in 2006 and 2007, but said to the best of its knowledge fewer than 10 clients were involved.

HSBC "unreservedly apologised" to clients for the threat to their privacy on Thursday, but said Swiss authorities said they will not support the use of the stolen data to answer requests from foreign authorities about tax issues.

Falciani is also reported to have attempted to sell the data to Germany for 2.5 million euros (2.27 million pounds), which tax investigators there estimated could rake in 100 million euros for German coffers.

Alexandre Zeller, CEO of HSBC's private bank in Switzerland, said Falciani transferred data onto a computer other than the one the bank issued him and fled to neighbouring France while under investigation.

Data theft risk was endemic to the business, Zeller said, adding that the bank had already added protection against this with USB keys and other technologies.

"We will always have new threats and we always have to invest more to ensure the IT security is all it can be," Zeller said. "At the end of the day, whatever you do, the human factor will remain. Private banking is a business of trust."

The stolen client information is limited to accounts in Switzerland, excluding ex-HSBC Guyerzeller accounts.

(Additional reporting by Douwe Miedema and Jason Rhodes; Editing by Rupert Winchester)