A French banking watchdog announced on Wednesday it had fined the French branch of Swiss bank UBS €10 million for helping hundreds of well-heeled clients stash money away in undeclared Swiss accounts.
The bank immediately hit back in a statement, protesting what it said was a disproportionate fine.
It also evoked "the contentious nature of the decision," and said it would lodge an appeal before the Council of State, which acts as the supreme court for administrative justice.
UBS France has been placed under formal investigation for trying to persuade rich clients to evade taxes through dodgy accounts in the small Alpine nation.
The ACP, the Bank of France's regulatory arm, had been alerted to the parallel accounts that ran between 2002 and 2007. They were not declared in France.
The investigating magistrates handling the UBS affair have sent a list containing 353 names of people suspected of having held a Swiss account and have requested details from the Swiss authorities.
The ACP on Wednesday said the bank had been "lax" in taking corrective measures, adding that the management "had waited 18 months to put controls to rectify this trans-border activity."
The French probe was launched after allegations from a former UBS employee turned whistleblower.
The issue jumped back to the top of the government's agenda in the wake of a scandal surrounding the former budget minister Jerome Cahuzac who in April was himself placed under investigation for tax fraud.
After months denying the allegations, Cahuzac had admitted to opening an undeclared Swiss bank account in 1992, and, after Switzerland pledged to cooperate with foreign tax authorities in 2009, transferring the some €600,000 to Singapore.
His role as budget minister included tackling tax evasion.