PARIS (REUTERS) - Swiss bank UBS has been placed under formal investigation for alleged complicity in suspected illegal sales practices, the French prosecutor's office said on Friday, a week after the bank's French business suffered the same fate.

At issue is whether UBS offered potential French clients investments that prosecutors allege were designed to evade taxes.

The move is the latest sign of a widening of the French investigation into UBS which is part of a global crackdown on tax evasion following the financial crisis.

The resignation of former French budget minister Jerome Cahuzac over allegations he had an undeclared Swiss bank account has bolstered political concern over the issue. Mr Cahuzac admitted to holding a Swiss bank account in March.

UBS was also given the status of "supervised witness" on two other allegations related to money laundering and tax evasion rather than being placed under investigation, the prosecutor's office said.

Under French law, being placed under official investigation means there exists "serious or consistent evidence" pointing to probable implication of a suspect in a crime. It is one step closer to a trial, but a number of such investigations have been dropped without trial.

Supervised witness is a lesser status which means that a person must be accompanied by his or her lawyer if questioned further in the probe.

UBS's French unit was placed under investigation a week ago after prosecutors had questioned the unit's CEO Jean-Frederic de Leusse.