Berlusconi is already facing charges of fraud and corruption [EPA]

Silvio Berlusconi, the Italian prime minister, is facing a prostitution investigation over allegations that he improperly assisted a teenaged nightclub dancer who attended parties at his private residence, prosecutors have said.

The announcement comes a day after Italy's constitutional court partially struck down legislation that gave Berlusconi blanket immunity from prosecution.

The embattled politician is already facing fraud and corruption charges, while he struggles to shore up his government's position in the parliament.

Magistrates are examining whether Berlusconi intervened improperly with police to secure the release of 17-year-old Karima El Mahroug, who uses the stage name "Ruby", after she was held over theft allegations. 

"Ruby" case

The investigation also covers allegations of underaged prostitution. The age of consent is 14 in Italy but exploiting or favouring prostitution of minors aged under 18 is a crime.

The alleged crimes took place between February and May 2010, according to a statement from prosecutors, and summons have been issued to Berlusconi and his lawyers.

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

The "Ruby" case caused an international media storm last year. The girl told newspapers that she had been paid 7,000 euros after she attended a party at Berlusconi's residence near Milan, but she 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.

His position is that he was simply offering assistance to a person in need, and that he was not using any improper influence or pressuring officers to let her go.

"The reality ... is that Berlusconi certainly placed a call to that police station. He claims, however, that he only called the police station to ask about her and to see if she was a relative of Egyptian president Hosni Mubarak," Sabina Castelfranco, Al Jazeera's correspondent in Rome, reported.

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