Apr.15, 2010, 8:44PM BST

French prosecutors have launched an investigation into possible money laundering by HSBC private banking clients after buying data stolen by a former employee of the bank.

Eric de Montgolfier, the prosecutor from Nice in the south of France, revealed yesterday that his team are looking into thousands of accounts for evidence of money laundering, having already passed on the details to tax authorities to establish whether the wealthy individuals had avoided paying tax.

The French have the details of clients of HSBC's Swiss private bank after purchasing a stolen disk from Herve Falciani, a former HSBC computer specialist. Mr Falciani is living in France and risks arrest if he leaves the country.

Mr de Montgolfier said he was investigating 8,231 French customers of HSBC who had accounts in Switzerland and accounts held by around 7,000 Italians. In total, he claimed the disk contained 127,000 HSBC accounts held by 79,000 people from various countries.

A spokesman for HSBC said those numbers were "massively overstated". "I have no idea where the figures come from," he added. Europe's biggest bank last month said the data theft affected up to 24,000 Swiss client accounts – only 15,000 of which are current.

The prosecutor could press charges against the French HSBC customers if he felt they had dodged tax in France and if there was evidence of money laundering. Governments around the world are looking to clamp down on money laundering and tax avoidance in the aftermath of the global financial crisis.

HSBC said it had lost only very few clients since the disk was sold. Swiss bank UBS has seen billions of dollars withdrawn since US authorities investigated it over tax avoidance practices.