London, Jan 14 (Agencies): A judicial enquiry has started into Italy's controversial Prime Minister Silvio Berlusconi's dealing with a 17-year-old nightclub dancer, amid allegations of underage prostitution, media reports said on Friday.

Investigators are reportedly examining whether the 74-year-old Italian premier intervened improperly to have Karima El Mahroug, a Moroccan nicknamed Ruby, released from a police station last May after she was held over theft allegations.

The Italian leader is suspected of abusing his power in trying to cover up his alleged encounters with Ruby, according to Corriere della Sera, an Italian daily newspaper.

The judicial enquiry also allegedly involves under age prostitution allegations, the report published in the Milan daily said.

Prosecutors in Milan were quoted as saying by the Guardian daily that Berlusconi has been formally placed under investigation on suspicion of prostitution.

A statement by the prosecutors said Berlusconi, who has not been charged, had been invited to present himself for questioning. The prosecutors said he had been formally made a suspect on December 21, but the news only broke on Friday on the website of the Italian daily Corriere della Sera.

The British Daily Telegraph cited the report as saying that the prime minister is to face a prostitution probe over the teenage nightclub dancer who attended parties at his private residence.

Even as Berlusconi has acknowledged knowing Ruby, who has told newspapers she was paid 7,000 euros after she attended a party at his residence near Milan, the premier has denied any wrongdoing in the case. He has denied using any improper influence or pressuring  officers to let her go.

Italy's constitutional court on Thursday opened the way for corruption trials involving  Berlusconi to resume after modifying a temporary immunity law that has shielded the Italian leader.

The court partially overturned a law that gave the prime minister immunity from three corruption and tax fraud trials linked to his multi-billion pound business empire, the report said.

According to Berlusconi's legal team, the investigation was "absurd and groundless" and the allegations raised had already been denied by the main figures in the case. They called the inquiry a "very serious interference in the private life" of the prime minister.