HSBC has confirmed that a theft of data by a former employee affected up to 24,000 Swiss client accounts.
The bank had previously said "less than 10 clients" were compromised after a former HSBC computer specialist stole client data from the bank which he handed over to French tax authorities.
HSBC said in a statement: "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."
Meanwhile up to 9,000 accounts that had been closed in the past were also affected.
These accounts often were not big enough to be eligible for private banking services, the bank said. It has 100,000 clients in Switzerland.