HSBC Holdings Plc’s Swiss private bank said it suffered “serious data theft” affecting about 15,000 Swiss-based accounts.

The data was stolen by a former information technology employee about three years ago, the Geneva-based unit of HSBC said in a statement. French authorities, which seized the data, have told their Swiss counterparts that they won’t use the information “inappropriately,” the bank said.

“The bank does not believe that the stolen data has or will allow any third party to access any client account,” HSBC said in the statement. The accounts were all opened before October 2006, the bank said.

"We deeply regret this situation and unreservedly apologize to our clients for this threat to their privacy," Alexandre Zeller, chief executive of the Swiss unit, told reporters.

The stolen data, which involves accounts opened at HSBC in Switzerland before October 2006, represents a far wider portion of HSBC's Swiss bank clients—roughly 15%—than the bank originally disclosed when it said in December the data had been stolen. At the time, the bank said it knew of fewer than 10 names potentially affected by the theft.

The willingness of governments to pay for stolen data has been in the spotlight after Germany bought a stolen disc containing names of 1,500 people with Swiss accounts who may have evaded German taxation. The move fanned tensions in Switzerland, which sees the purchasing of the disc as another attack on its cherished banking secrecy.

Earlier in the week the Bulgaria's revenue agency chief said a stolen disk of Swiss banking data bought by Germany has revealed that Bulgarian tax evaders held about EUR 200 M in banks in Liechtenstein and Switzerland.

Krasimir Stefanov arrived in the German capital Friday to consult the stolen disc containing the names of 1,500 Swiss account holders who may have defrauded their country's tax authorities.

Germany, which is said to have shelled out EUR 2.5 M for the disc, has agreed to share the data with Bulgaria, but it is still not clear whether it will have to pay for the information too.

About a dozen alleged tax fraudsters had already asked if they could voluntarily turn themselves in, according to local media reports.