Silvio Berlusconi has been summoned by magistrates to answer accusations of sexual misconduct and corruption.
News of the judicial probe came less than 24 hours after senior judges stripped the beleaguered prime minister and media mogul of his immunity from prosecution.
Mr Berlusconi is suspected of having sex with an under-age prostitute and abusing his power of office, according to court leaks reported by the Corriere della Sera newspaper.
The prostitute, Moroccan belly dancer Karima "Ruby" El Mahroug, then 17, was at the centre of the scandal that engulfed the prime minister in the Autumn when extensive claims emerged that he had organised what came to be known as "bunga bunga" parties at his mansion outside Milan, for which dozens of young prostitutes were coached in for after-dinner orgies.
The age of consent in Italy is 14. However, paying to have sex with someone under the age of 18 is an offence which carries a maximum three-year jail sentence. Ms El Mahroug has denied sleeping with Mr Berlusconi.
More serious, however, are claims by prosecutors that the prime minister abused his powers when he called Milan police on the night of 27-28 May to secure the release of Ms El Mahroug following her arrest for suspected theft. A conviction for abusing his office could carry a prison sentence of up to 12 years.
It was reported in October that Mr Berlusconi telephoned Milan police headquarters and said the Moroccan runaway was the granddaughter of the Egyptian President Hosni Mubarak. Milan's juvenile crime magistrate, Annamaria Fiorillo, later insisted that she had not given her permission for the release of the girl. She said she had told police at the time: "If she's the granddaughter of President Mubarak then I'm Queen Nefertiti."
The prosecutors allege that he intervened – and abused his powers – to make it less likely that she would spill the beans on his alleged X-rated soirees, for which young women were said to be paid €5,000 a time.
Mr Berlusconi's lawyers, Niccolo' Ghedini and Piero Longo, condemned the Milan magistrates' investigation, saying it represented a "very serious interference in the prime minister's private life that has no precedent in the country's judicial history".
"The new investigation of the prime minister by Milan magistrates appears so absurd and unfounded in terms of the facts and the law that it wouldn't even merit a comment," they said. However, the Ansa news agency reported yesterday afternoon that Mr Berlusconi had already been called by magistrates to appear for questioning later this month.
After the Constitutional Court crucially weakened Mr Berlusconi's immunity protection on Thursday, he was already facing the prospect of returning to court on other charges. Under the watered-down legislation, the prime minister can only avoid court appearances by citing government business with the permission of the trial judge.
In one trial due to restart, he is accused of bribing lawyer David Mills, the estranged husband of Britain's former Olympics minister Tessa Jowell, with $600,000 to lie under oath. He also faces charges of tax fraud. However, few people expect a definitive conviction on these charges or the "Rubygate" affair under Italy's convoluted legal system.