France is formally investigating Swiss bank UBS, which is suspected of helping wealthy clients open undeclared bank accounts in Switzerland.
An official at the prosecutor's office in Paris told the BBC the bank is suspected of "complicity in illegal sales practices".
A shadow accounting system was reportedly set up that masked transfers between French and Swiss bank accounts. UBS said it was cooperating.
The bank's French unit was also formally placed last week under scrutiny on similar suspicions.
The former head of UBS France and two other executives in Lille and Strasbourge are already being investigated.
The BBC reports the accusations came to light after former staff blew the whistle on practices that involved hundreds of retail and corporate clients.
An anonymous letter was reportedly sent to the regulatory arm of the French central bank suggesting that parallel accounts were opened in Switzerland but undeclared in France, which is illegal under French law.
The letter said a special record was kept between 2002-2007 listing undeclared bank accounts that had been opened by corporate clients.
