ROME - Prosecutors are investigating whether Premier Silvio Berlusconi had sex with a teenage prostitute and then abused his power in trying to cover up the encounters with the girl, officials said Friday.

The investigation, which Berlusconi's lawyer called "absurd and groundless," escalated a long history of accusations of sexual and financial impropriety aimed at the billionaire businessman, who has shaken off the crises and maintained his hold on power.

The latest allegations are tawdry even for scandal-jaded Italians, however, and they come at a moment of particular vulnerability for the blunt-speaking, high-living prime minister. Berlusconi 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.

According to a statement by Milan prosecutors, the probe is looking into whether the 74-year-old premier had sex with a 17-year-old nightclub dancer nicknamed Ruby by the press, and then used the powers of his office inappropriately in trying to hide the crime.

The prosecutors issued a summons Friday for Berlusconi and his lawyers, the statement said. They also ordered Milan police to search the offices of various people implicated in the case, including a showgirl-turned-politician who is close to the premier.

Berlusconi's attorneys, Nicolo Ghedini and Piero Longo, said 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 the allegations "have already been refuted by all witnesses and people directly involved."

In recent years, Berlusconi has been engulfed in a series of scandals relating to his private life, including reports of wild parties at his mansions and alleged encounters with two other prostitutes. In 2009 his second wife, Veronica Lario, announced she was divorcing him, citing Berlusconi's presence at the birthday party of an 18-year-old model and his fondness for younger women.

Berlusconi has made no apologies for his lifestyle, saying recently, "I love life, I love women!" In the past he denied having paid to have sex, though he also said, "I'm no saint."

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. According to unsourced press reports at the time, Berlusconi had told the police she was a relative of Egyptian President Hosni Mubarak.

When the scandal broke, Berlusconi said he had done nothing wrong, and that he had only intervened to help someone in need. He insisted he called Milan police solely to inform them that somebody available to act as the girl's guardian was going to the police station where Ruby was being held.

The person who went to the police on Berlusconi's behalf is Nicole Minetti, the former showgirl whose offices were searched by police. Minetti, who was elected in a regional assembly in Milan for Berlusconi's party, is also under investigation, the prosecutors said.

Ruby said in newspaper interviews months ago 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 age of consent for sex in Italy is 14, but exploiting or aiding prostitution of minors under 18 is a crime. The girl has in the meantime turned 18.

The prosecutors are investigating whether Berlusconi called Milan police on the night of May 27-28 to secure the release of the girl because he wanted to hide the fact that he had been the girl's client during encounters at his villa at Arcore, near Milan, between February and May, according to the statement.

Berlusconi was placed under investigation Dec. 21, the prosecutors said. They confirmed the probe Friday after Italy's leading newspaper, Corriere della Sera, reported on it.

Many of the premier's political allies said the latest probe was yet another attempt to hurt the premier by what they say are left-leaning magistrates.

"If someone ... still believes that Silvio Berlusconi can be defeated through judicial means, once again they will be sorely disappointed," said Daniele Capezzone, a spokesman for Berlusconi's party.