Budget Minister Jerome Cahuzac said he will sue the investigative news website
Mediapart, over its allegation that he held such an account with Swiss bank UBS
Amid rising speculation about Cahuzac’s position, on Thursday, Prime Minister Jean-Marc Ayrault stood behind him, saying there was no question of him leaving the government.
"I have every confidence in Jerome Cahuzac and I can imagine what he is feeling right now," Ayrault told RTL radio. "He has started to fight back, it's difficult, but he will fight for his honour."
President Francois Hollande and other ministers have all supported Cahuzac, who described the allegations against him as "crazy".
The allegations are bound to be troubling, however, for the government of Hollande, who made ministers sign an ethics code and vowed to rule with integrity when he took office in May.
Right-wing parliamentarian, Laurent Wauquiez, an ally of former president Nicolas Sarkozy, said that the burden of proof lay with Cahuzac, and that he should produce documents to support his denials.
Cahuzac has been leading efforts to fight tax evasion as the Socialists seek to eliminate France's budget deficit and revive the country's struggling economy.
Last month he launched a campaign aimed at bringing in a billion extra euros in tax revenues next year.
Mediapart says its report was based on several witness accounts and documents showing that Cahuzac owned the Swiss bank account, undeclared to French tax authorities, for several years before transferring the funds to a Singapore account in 2010.
Mediapart released an audio recording, in which a man they claim is Cahuzac admits
to having the account. He is heard saying: "It bothers me to have an account there, UBS is not necessarily the most hidden of banks."
The news website said it confirmed the information with a source at UBS and with a former tax official, who had raised the existence of the account in a memo to his superiors in 2008.
Cahuzac said the evidence presented by Mediapart was neither "impressive" nor "convincing".
"I do not have, and have never had, a bank account in Switzerland or elsewhere abroad," he said. "No credible witness can assert or try to corroborate something that does not exist and has not existed."
Mediapart is known for its investigative reporting and in 2010 ran a story linking former president Nicolas Sarkozy to allegations of illegal party campaign donations by L'Oreal heiress Liliane Bettencourt.
Sarkozy was questioned for 12 hours last month by judges investigating the accusations but was not charged.
Little-known before being named a minister, Cahuzac, 60, began his career as a cardiologist before switching to specialise in plastic surgery, in particular hair transplants.
Elected to the National Assembly in 1997, he headed its commission on public finances and in June, was chosen by François Hollande to be budget minister.
French media reported in October that Cahuzac's 300-square-metre apartment in Paris's exclusive 16th district was robbed, with thieves taking 100,000 euros worth of luxury watches.