French prosecutors opened a formal tax fraud investigation on Tuesday into allegations that Budget Minister Jerome Cahuzac held a secret bank account in Switzerland.
Cahuzac, who is leading a government crackdown on tax evasion, 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.
However the public prosecutor said police laboratory tests showed a correlation between the voice of Cahuzac and that in a recording of a telephone call published by Mediapart, in which 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.
There was no immediate comment from the government.
French prosecutors opened a preliminary investigation into the affair in January, but this step takes the inquiry to a new level and will involve deploying greater resources to the case and cooperation with other judiciaries, notably in Switzerland.
Cahuzac, a former plastic surgeon who rose to prominence as the Socialist Party's toughest budget-watcher, has led efforts to crack down on tax evasion and fraud by French citizens seeking to avoid high levies at home.
If he were to be formally charged with tax fraud, Cahuzac could have to step down from his post and pass on responsibility for pushing through a tough 2013 budget that raises taxes and freezes public spending.