HSBC confirmed that a former employee stole client data but said the number of records taken was less than 10.
The theft highlights ongoing problems that companies often have with their own employees, who may steal sensitive data for later profit or revenge before their employment is terminated.
The data has ended up in the hands of French authorities, which have been investigating up to 3,000 people thought to be avoiding taxes, according to French media reports.
The records were stolen around the end of 2006 or in early 2007 by an employee who worked in the IT department, according to an HSBC spokesman.
HSBC filed a criminal complaint in 2008, and the person was questioned by Swiss prosecutors and has since been charged in the theft, the spokesman said.
The data came from HSBC Private Bank, which offers consulting and banking services to wealthy clients. All of the affected customers have been notified, the spokesman said. Their collective undeclared wealth was estimated at €3 billion (US$4.4 billion), according to a report in The Connexion, an English-language newspaper in France.
French Budget Minister Eric Woerth said earlier this week the employee gave the information to the French government, where it was verified to see if it was reliable, according to The Connexion. Woerth said the government did not pay for the data.