The legality of a French crackdown on suspected tax evaders earlier this year has been thrown into doubt after it emerged that stolen data was among the mix of information used by financial investigators.

A list of 3,000 French nationals suspected of using Swiss banking secrecy to evade paying taxes included data handed over by a former IT worker for HSBC in Switzerland - without the bank's permission - to the French authorities.

In a statement, HSBC in Switzerland confirmed a worker suspected of stealing information from the bank between 2006 and 2007 was prosecuted last year. The data involved less than 10 accounts held by Geneva-based HSBC Private Bank, according to HSBC. It's unclear whether the unnamed worker involved was convicted of any offence. French daily Le Parisien reports that the former bank staffer has fled to France and is living under judicial protection.

French daily Le Figaro claimed on Friday that up to 4,000 French clients of the bank, collectively holding €6 billion ($8.8 billion) in assets in Switzerland, were named on the stolen list. Only an unspecified proportion of those named on the list (which sounds like a data dump, perhaps indexed by a residential address in France) are suspected of tax evasion.

Eric Woerth, French budget minister, said this week that the tax evasion list was compiled from "several sources, not only that one", a reference to the list of suspect provenance involving HSBC accounts, the FT reports. Woerth added that the government did not pay for the HSBC data.

The Minister went on the say that French authorities are in the process of going through 1,400 dossiers, which they hope might bring in €500 million ($730 million) in tax revenues, according to Swiss reports.

More on the ongoing financial mystery can be be found in reports in The Times here and elsewhere here. ®