Magistrates last week placed UBS France under investigation and now want to know if UBS, based in Zurich, illegally solicited clients in France, The Wall Street Journal reported.
French banking customers can only be signed up by banks registered in France.
UBS has pledged to cooperate with the investigation.
"UBS does not tolerate any activities intended to help its clients circumvent their tax obligation," a bank spokesman said.
The bank will "continue working with the authorities in France within the applicable legal framework to arrive at a resolution to this matter," the spokesman said.
Two former UBS France executives and a current executive were investigated last year on allegations of money laundering and tax evasion.
UBS paid a $780 million fine four years ago, after admitting to U.S. authorities it helped U.S. citizens use Swiss bank accounts to evade taxes.