GENEVA (Bloomberg) — HSBC Holdings Plc's Swiss private bank said a former employee stole details on 15,000 existing accounts, as banking secrecy comes under growing pressure from nations keen to crack down on tax evasion.

An information technology worker took the account information about three years ago, the Geneva-based unit of HSBC said in a statement yesterday. Data were also stolen on 9,000 accounts closed before October 2006, said the bank, which currently has about 100,000 accounts in all.

"This represents a threat to the privacy of our clients," Alexandre Zeller, chief executive officer of HSBC's private bank, told reporters today in Geneva. "We deeply regret this situation and unreservedly apologise to our clients."

The bank plans to spend 100 million Swiss francs ($93 million) on improving security, he said.

French authorities, who obtained the data, have told their Swiss counterparts that they won't use the information "inappropriately," HSBC said.

German Chancellor Angela Merkel said last month that her government may buy stolen data on Swiss bank accounts, two years after Germany paid 5 million euros ($6.8 million) for details filched from LGT Group in Liechtenstein.

The willingness of governments to pay for stolen data is fanning tensions with Switzerland's neighbours as it seeks to negotiate treaties committing it to cooperate with international tax probes. The Swiss government said in January it will draft a law barring officials from assisting foreign countries in cases involving theft of client details. "This is enormous and no-one expected that it could happen to HSBC so it's a tough lesson for the whole industry," says Bernhard Bauhofer, founder of Sparring Partners GmbH, which advises companies on managing their reputations. "There's an increasing demand for data and there will be other cases because governments are looking for funds; where there's demand there will be supply," he said.

The French Finance Ministry said in December that it had data on Swiss bank accounts held by French taxpayers, including names provided by a former HSBC employee.

Switzerland suspended treaty negotiations with France in December because of the HSBC case. After talks in January, France agreed to return the original data to Switzerland and not ask for assistance from Swiss authorities based on the stolen information. France will continue to use the data to pursue tax evaders at home.

"The bank does not believe that the stolen data has or will allow any third party to access any client account," HSBC said. The accounts were all opened before October 2006, the bank said, adding that it is contacting all clients with Swiss-based accounts.