French authorities are formally investigating UBS for allegedly helping wealthy clients open undeclared bank accounts in Switzerland.
It also allegedly set up a shadow accounting system that masked transfers between French and Swiss bank accounts.
UBS said it was cooperating with authorities.
Under French law, being placed under official investigation means there is 'serious or consistent' evidence to implicate a suspect in a crime. But it does not necessarily lead to a trial, the official said.
The latest move follows action taken against the bank's French unit last week which was also formally placed under scrutiny on similar suspicions.
The allegations originally came to light after former staff blew the whistle on the 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.
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