Herve Falciani, a former HSBC computer specialist, has admitted stealing client data from the bank which he passed to French tax authorities.
The company has previously claimed that "less than 10 clients" had been impacted by the theft, but now admits many more were hit.
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"It is now clear that the theft, which was perpetrated by a former IT employee about three years ago, involves approximately 15,000 existing clients who had accounts with the bank in Switzerland before October 2006," HSBC said.
The theft triggered a brief spat between Switzerland and France last year after French authorities recovered the data from the former employee to probe suspected evasion by French taxpayers who put money into Swiss accounts.
HSBC said on Thursday that Swiss authorities said they would not support use of the stolen data to answer requests from foreign authorities about tax issues.
The stolen client information was limited to accounts in Switzerland, excluding ex-HSBC Guyerzeller accounts.
