The US prosecution of the bank lead to a $780 million settlement two years ago.
Birkenfeld, an American, was implicated in the previous UBS case and given a prison sentence in 2008 for conspiring to defraud the US. He remains under supervised release and requires approval from the court to travel overseas.
French judges have been investigating whether UBS solicited clients in France to open Swiss accounts used to hide assets, and last December the bank lost an appeal against a ruling requiring it to post a 1.1 billion ($1.6bn) bond as part of that probe.
UBS has said previously the bond amount is “exaggerated, speculative and not based on facts”, and says it plans to challenge the bond decision at the European Court of Human Rights.
Birkenfeld said he had been asked to assist the investigation by providing testimony and documents.
When assisting the U.S. Department of Justice Birkenfeld he outlined in detail detailing how the bank actively helped clients maintain undeclared accounts shielded by Switzerland’s bank secrecy laws.At the time of its settlement, UBS acknowledged helping Americans evade taxes.
Jeffrey Newman represents whistleblowers. He does not represent Mr. Birkenfeld.