Magistrates have placed the French unit of UBS under formal investigation for alleged witness tampering after a former employee filed a complaint against the bank, Paris prosecutors said.

French prosecutors are probing whether UBS France, which faces preliminary charges for its alleged complicity in tax fraud, tried to silence Nicolas Forissier, the bank's internal auditor between 2001 and 2009, after he alerted his management and French authorities about the bank's alleged practises.