Silvio Berlusconi pay for sex.

A teenage Moroccan model has claimed she slept with Silvio Berlusconi, the Italian prime minister, in return for a cash "gift", according to reports.

(Silvio Berlusconi, the Italian prime minister)

The girl, identified only as Ruby and allegedly an illegal immigrant, claims that she had consensual sex with the prime minister at his luxury villa in Arcore, outside Milan, according to Italian press reports.

Prosecutors have reportedly opened a dossier on the claims but not a formal investigation. They stressed that Mr Berlusconi is not under investigation and that they are trying to verify the woman's account of the alleged encounter

But Il Giornale, the newspaper owned by Mr Berlusconi's brother, described the allegations as a potential "sex scandal" and another "D'Addario case" – a reference to Patrizia D'Addario, the prostitute who last year claimed that she slept with the prime minister at his mansion in Rome.

Magistrates cautioned that the Moroccan woman's claims might be without foundation and could be part of a plot to blackmail the 74-year-old premier, or an attempt at self-promotion by the aspiring showgirl.

The teenager, who is now 18, claims that she attended several private parties thrown by Mr Berlusconi at his home between last year and this Spring, at a time when she was trying to break into modelling or television.

She was part of a circle of aspiring models and actresses who were allegedly introduced to the prime minister by a prominent show business agent, Lele Mora, although he has denied all knowledge of the case.

Investigating magistrates said her accounts were sometimes contradictory and that she had made claims which she subsequently retracted.

The story emerged when she was thrown out of an apartment in Milan after an argument with her Brazilian flatmate in May, the Italian press reported.

She was stopped by police, who found she had no residence permit, but pleaded for leniency on the basis that she knew the prime minister, and proceeded to tell them of her alleged encounter.

One of the magistrates in charge of the case, Antonio Sangermano, has experience in investigating prostitution and extortion cases, but on Wednesday refused to comment on the claims.

Prostitution itself is not illegal in Italy, but the law prohibits organised prostitution and you must be older than 18.

A member of Mr Berlusconi's People of Freedom Party dismissed the story and predicted that it would prove to be untrue.

"This is the same old stuff – as we get closer to elections, out come the various D'Addarios bragging about their encounters with the prime minister," said Nunzia De Girolamo, an MP.

The allegations come at an awkward time for Mr Berlusconi, whose approval ratings are at their lowest level since he was re-elected prime minister in May 2008.

He has been involved in a feud with his former political partner, Gianfranco Fini, who has threatened to block any legislation that he and a group of rebel government MPs do not like.

Mr Berlusconi has for months tried to pass legislation that would guarantee him immunity from prosecution in two potentially damaging corruption trials in which he is accused of bribery, tax fraud and false accounting.

In one of the trials, he is accused of paying a £370,000 bribe to David Mills, the estranged husband of former Olympics Minister Tessa Jowell, in exchange for giving false testimony in court.