UBS France is suspected of trying to silence a whistleblower who was behind revelations of alleged tax fraud by the bank.
The French subsidiary was also designated an "assisted witness", a status between a full witness and being under formal investigation, on a charge of moral harassment.
UBS is alleged to have set up between 2004 and 2012 a system to incite rich French clients to open bank accounts in Switzerland to avoid paying tax at home.
Statements by former internal audit official Nicolas Forissier at UBS France contributed to revealing the affair.
Employed in 2001, Forissier was fired in November 2009.
In 2012 a court ordered UBS France to pay him 300,000 euros in compensation.
Four other former employees, who had alerted Forissier to the fraud or who had refused to destroy documents related to the case, also received compensation packages from UBS France.
According to Forissier's lawyers, William Bourdon et Apolline Cagnat, it's the first time a bank implicated in this scale of tax fraud has put pressure on the person who had originally leaked the information.
"Trying to silence the main whistleblower is not exactly a sign of calm and innocence," they said.
UBS has denied any wrongdoing and contests Forissier's version of the story.
German documents given to judges revealed that around 12 billion euros worth of assets were being held by some 38,000 French clients in the bank in 2008. Not all of the cases were found to be fraudulent.
Belgium on Friday charged UBS with "serious and organised" fiscal fraud for encouraging clients to cheat on their taxes, as well as being involved in money laundering.