The decision to open an investigation adds to a long series of legal problems hanging over the leader of Italy's main centre-right party who has been banned from parliament over a separate tax fraud conviction.
It may also complicate efforts to reform a complicated electoral system blamed for chronic instability in Italy, following an agreement at the weekend between Berlusconi's Forza Italia party and the centre-left Democratic Party.
Berlusconi was convicted last year of paying for sex with former nightclub dancer Karima El Mahroug, better known by the stage name Ruby the Heartstealer, when she was under 18 and of abusing his powers as then prime minister to get her released from police custody over unrelated theft accusations.
The 77-year-old media billionaire has appealed against the sentence, which will not take effect until the whole appeals process is completed.
Berlusconi repeated his accusations judges and prosecutors had targeted him for political reasons and said he would not be deterred.
He said: "I will remain in place, more convinced than ever of the need to keep fighting for what I believe in profoundly."
The latest investigation follows the conviction of three associates in a related trial in which they were found guilty of procuring young women for prostitution at parties at Berlusconi's luxurious villa near Milan.
Last year judges in the trial requested prosecutors to consider opening an investigation into allegations Berlusconi and his lawyers had met some of the young women for meetings to discuss the evidence they planned to give in court.