Silvio Berlusconi is facing a fresh investigation over a teenage nightclub dancer who attended parties at his private residence.
It comes a day after Italy’s top court partially struck down a law that gave him a blanket immunity from prosecution.
Magistrates will look into allegations that the prime minister paid to have sex with 17-year-old Karima El Mahroug, who used the stage name “Ruby Rubacuori” (Heartstealer).
Paying for sex with a prostitute under 18 is an offence in Italy.
Prosecutors are also examining whether he intervened improperly with police to secure her release after she was held for theft.