Premier Silvio Berlusconi is under investigation in a prostitution case involving a 17-year-old Moroccan girl, a case his lawyers on Friday called "absurd and groundless."

Italy's leading newspaper, Corriere della Sera, reported that the 74-year-old premier is suspected of having sex with the minor and then abusing his power in trying to cover up the encounters with the girl, nicknamed Ruby by the press.

Berlusconi's attorneys, Nicolo Ghedini and Piero Longo, confirmed the investigation hours later, saying probe represented a "very serious interference in the prime minister's private life that has no precedent in the country's judicial history."

They called the case media-driven, and said that the allegations "have already been refuted by all witnesses and people directly involved."

The investigation escalates a long history of accusations of sexual and financial improprieties aimed at Berlusconi, a billionaire businessman who has shaken off the crises and maintained his hold on power.

It comes, however, at a moment of particular vulnerability. The premier has been politically weakened in recent months and a law shielding him from two, unrelated trials in Milan was significantly watered down Thursday by a ruling of Italy's Constitutional Court.

The Ruby case became public months ago when it emerged that Berlusconi had intervened to secure the release from police custody of the girl, who had been held for theft.

Berlusconi said at the time that he had done nothing wrong, and that he had only intervened to help someone in need. The girl said in newspaper interviews that she had been at the premier's villa and that she lied about being over 18. She denied having sex with the premier.

The girl has in the meantime turned 18.

Berlusconi's office said it had no immediate comment. Many of his political allies said the latest probe was the latest attempt to hurt the premier by what they say are left-leaning magistrates. The office of Milan prosecutor Antonio Sangermano said he was not available for comment.

According to Corriere, the prosecutors allege that Berlusconi abused his power when he called Milan police on the night of May 27-28 to secure the release of the girl. The prosecutors believe he did so because he wanted to hide the fact that he had been her client, according to the report, which did not cite sources.

The ANSA news agency said Milan prosecutors on Friday summoned Berlusconi. Reports also said police on Friday searched the offices of two of his aides.

Daniele Capezzone, a spokesman for Berlusconi's party, said the case was politically motivated.

"If someone ... still believes that Silvio Berlusconi can be defeated through judicial means, once again they will be sorely disappointed," he said.