Jérôme Cahuzac, who was forced to quit his cabinet-level job as budget and tax enforcement minister last year, already has confessed to having kept a clandestine account in Switzerland.
The allegation about the Isle of Man account comes from Cahuzac's wife, Patricia Menard, who is divorcing him, according to media reports, which have included a story in the Daily Mail as well as French publications. The Daily Mail said lawyers for the Cahuzacs "would not comment on the latest revelations", which it said had been confirmed by legal sources close to the investigation.
Menard has alleged that the account had been opened between 1997 and 2004, in order to channel €2.5m ($3.48m, £2.09m) into London's property market.
The investigating magistrates in France have set up an enquiry in London to try and trace these investments, the Daily Mail report, which quotes France's Sud Ouest newspaper, said.
According to the reports, the complexity of the evidence trail means that Cahuzac's trial is not expected to take place until next year at the earliest.
Isle of Man: 'open jurisdiction'
A spokesman for the Isle of Man said the jurisdiction had nothing to hide, and was in full compliance with existing standards for transparency and regulation.
John Spellman, director of financial services, Department of Economic Development, said Manx officials couldn't comment on individual cases. But "we have signed up to reporting standards and exchange protocols," he added. "We are not trying to hide anything. We are an open jurisdiction, with no banking secrets".
Account unveiled in 2012
Cahuzac's account with Swiss bank UBS was exposed by the French news website Mediapart in December 2012. He sued for defamation, but was subsequently forced to confess its accuracy and resign.
Prosecutors found that the undeclared account had contained €600,000 when it was secretly moved to Singapore in 2009.
French media reports called attention to the irony of a minister in charge of ensuring others paid their fair share of tax who kept some of his own money stashed offshore.
In 2013, French government ministers were ordered to reveal details of their personal wealth, as part of efforts by French president Francois Hollande to regain public trust, in the wake of the news of Cahuzac's Swiss bank account. Under Hollande's orders, the data was subsequently posted on a website created for the purpose.
The Cahuzac scandal is also said to have inspired a package of stringent new anti-avoidance measures that came into force in France last June.
