Italian Prime Minister Silvio Berlusconi faces a prostitution investigation in Milan over a teenage nightclub dancer who attended parties at his private residence, prosecutors said Friday.

A day after Italy''s top court partially struck down a law that gave him blanket immunity from prosecution, the probe added pressure on the premier, who is battling to shore up his shaky government and already faces tax fraud and corruption charges.

The magistrates said they are investigating allegations that Berlusconi 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 age 18 is an offence in Italy.

They are also examining whether he intervened improperly with police to secure her release after she was held for theft.

Prosecutors said in a statement they were investigating alleged crimes committed at Berlusconi''s Arcore villa near Milan between February and May 2010, when the girl was 17, and that they had issued summons to Berlusconi and his lawyers.

The prime minister''s lawyer Niccolo Ghedini issued a statement dismissing the investigation as "absurd and groundless" and said the allegations had already been denied by witnesses and those directly involved.

The case of "Ruby" caused an international media storm last year and instantly made the expression "bunga bunga" - a term used by the young woman to describe sex parties - part of the Italian vocabulary.

She told newspapers she was paid 7,000 euros ($9,414) after attending a party at Berlusconi''s villa but denied having sex with him.

Berlusconi, who has weathered a series of sex scandals involving escorts and young women since returning to power in 2008, has acknowledged knowing Ruby and making a phone call to police on her behalf.

But he says he was merely offering normal assistance to a person in need and has denied pressuring officers to let her go. He also says nothing improper has ever happened at his parties.

Berlusconi was placed under investigation last month but the probe was only made public Friday.