French prosecutors have opened a formal investigation into allegations that Cahuzac held a secret bank account in Switzerland.

Cahuzac has repeatedly denied a report in December by French investigative news website Mediapart that he held an undisclosed account at the Swiss bank UBS until the start of 2010. Mediapart said he then transferred the money in the account to Singapore.

Prosecutors said on Tuesday that police laboratory tests showed a correlation between the voice of Cahuzac and that in a recording of a telephone call published by Mediapart. In the recording, a male voice acknowledges holding an account at UBS.

"In other words, the result of our analysis reinforces the hypothesis that Jerome Cahuzac is the unidentified speaker," the office of the Paris prosecutor said in a statement.

A preliminary investigation into the case had been opened in January. The investigation is now to be stepped up and include co-operation with other judiciaries, notably in Switzerland.

Cahuzac, a former plastic surgeon, had led efforts to crack down on tax evasion and fraud by French citizens seeking to avoid increased tax rates in France.

In a statement, Cahuzac said he had resigned to make sure the case did not interfere with the functioning of the government. He repeated : "I am innocent of the slanderous allegations made against me."

President Francois Hollande announced that Minister for Europe Bernard Cazeneuve would take over Cahuzac's brief as part of a reshuffle of his cabinet.

Thierry Repentin, a junior minister with responsibility for professional training, is to be placed in charge of France's relations with the rest of the European Union.

jm/mz (Reuters, AFP)