Rome prosecutors earlier in the week launched an investigation into whether the premier allegedly abused his powers by arranging for partygoers — including a dancer and singer — to use government aircraft and for official security escorts to travel to his villa on the island of Sardinia.

Giovanni Ferrara, the prosecutor leading the probe, told reporters he and his colleagues had launched the investigation after seizing hundreds of photographs that depict guests attending parties at the cactus-lined Villa Certosa, where Mr. Berlusconi often hosts foreign leaders with lavish dinners and shows.

Mr. Berlusconi said “all of these people were hosted at Certosa without a euro of cost to the state.” He was speaking on a morning talk show on Italy’s Canale 5 channel, which is part of the Berlusconi family’s media company Mediaset SpA.

The investigation adds yet another chapter to a string of controversies surrounding Mr. Berlusconi’s personal life — and its frequent overlap with his public role.

The premier’s wife, Veronica Lario, recently announced she was seeking a divorce from Mr. Berlusconi after accusing him of “consorting with minors” and of drawing telegenic young women into his political ranks. Mr. Berlusconi has also been under the microscope for his ties to an 18-year-old aspiring model. Mr. Berlusconi, who was photographed at the woman’s birthday party in April, has denied having a “spicy” relationship with her.

It is unclear whether the latest investigation will dent his popularity with Italian voters. Over his 15-year political career, Mr. Berlusconi has been under investigation — accused of crimes including corruption and false accounting — a number of times without the cases diluting his appeal. He has always maintained his innocence.

This week, polls show Mr. Berlusconi’s right-wing political coalition with a strong lead in the weeks leading up to elections for the European Parliament, which end Sunday.

Mr. Berlusconi has been acquitted in some trials against him. In others, charges were dropped when the statute of limitations on the cases ran out during appeals. Last year, Mr. Berlusconi pushed a new law through Parliament that grants immunity from criminal prosecutions to Italy’s top officials — himself included.

In the U.K. a scandal involving widespread abuse of parliamentary expenses is expected to help send the ruling Labour Party to defeat in the European elections. Almost 20 members of Parliament from across the political spectrum have stepped down or said they won’t stand at the next election after making expense claims on everything from toilet seats to payments on mortgages that no longer existed.

The Rome investigation stems from hundreds of unauthorized photographs snapped by an Italian photographer depicting parties that Mr. Berlusconi threw at Villa Certosa in 2008. A handful of the photographs have trickled into Italian newspapers and magazines. Prosecutors seized all the photos over the weekend after an attorney for Mr. Berlusconi filed a complaint alleging that the photos violated the prime minister’s privacy.

After reviewing the photos, Rome prosecutors opened an investigation earlier this week into whether Mr. Berlusconi abused his powers as prime minister, according to Mr. Ferrara. Mr. Ferrara said his office was seeking additional information, including the travel logs and passenger lists of government planes traveling between Sardinia and Rome.

Berlusconi Defends Flying Guests on Jet

On Thursday Berlusconi issued yet another denial of involvement with an 18-year-old model, telling reporters in Rome that if someone asked whether he had “a relationship, let’s say steamy or more than steamy, with an underage girl”, his answer would be: “Absolutely not.”

“I have sworn it on the life of my children and I said that I am aware that, if this were perjury, I would have to resign a minute later,” he said.

It seems that, no matter what he does these days, the Italian prime minister simply cannot keep his family either out of politics or off the front pages. At the beginning of this month, his wife, Veronica Lario, ignited a political bombshell when she announced that she wanted a divorce.

Not only did Mrs Berlusconi complain about the glamorous “starlet” candidates that her husband’s Freedom Party (PDL) intended to field in the European elections, but she also expressed her consternation at media reports that her 72-year-old husband had attended the 18th birthday party of Neapolitan Noemi Letizia.

Pointing out that he had failed to attend the 18th birthdays of the three children he had with her, Mrs Berlusconi said she did not want to be married to a man who “consorts with minors”.

Thus was born the caso Noemi which has seen the prime minister struggle to explain just how he came to attend the young woman’s 18th birthday party.

Mr Berlusconi even went on television to offer his own version of events, commandeering an edition of state broadcaster RAI’s evening current affairs programme, Porta A Porta, to explain that he had attended the party as a favour to her father, an old acquaintance.

The problem here is that no one can provide reliable evidence of any relationship between this ordinary Neapolitan family and Mr Berlusconi. Mr Letizia is a local government employee and small shop owner in the Secondigliano area of Naples.

Initial reports from Mr Berlusconi’s staff suggested that Noemi’s father was known to the prime minister because he had once worked as chauffeur to a former prime minister, the late Bettino Craxi, a long-time political ally and friend of Mr Berlusconi.

However, his son, Bobo Craxi, immediately issued a statement pointing out that Letizia had never worked for his father.

In an interview with Rome daily La Repubblica, Noemi’s former boyfriend Gino Flaminio offered an explanation.

Noemi had been ambitious to get started in showbusiness and had accordingly prepared a portfolio and CV which had been subsequently sent to various acting agencies. There, he claimed, she was spotted by senior journalist Emilio Fede, a faithful Berlusconi supporter and the chief news editor of one of his three commercial TV channels, Rete 4.

Fede had been looking for new talent for the channel, for female presenters for anything from the weather to news programmes.

Somehow, suggested the ex-boyfriend, Noemi’s portfolio had come to the attention of Berlusconi, who then rang her on her mobile phone. (He claimed to have listened in to some of the conversations and to have recognised the prime minister’s voice.)

Berlusconi, he said, had been paternal in his attitude, wanting to know what sort of studies Noemi was following. Flaminio went on to claim that the prime minister had invited Noemi and about 30 other girls to spend the new year holiday in one of his Sardinian villas.

Berlusconi’s supporters have questioned the validity of Flaminio’s testimony, pointing out that he has a criminal record. (He was given a suspended 2½-year sentence for the theft of a mobile phone). Others, from Nobel Prize winner Dario Fo to thousands of bloggers, have been less sceptical.

Hence, Dario Franceschini’s question, which prompted the angry reaction of all the Berlusconi children who claimed that he had been a good father and a man who had instilled “values” in all their lives.

However, Franceschini’s question will shortly face a far more serious test at the European ballot box.

If opinion polls are anything to go by, though, blog outrage, international press condemnation and even some (admittedly timid) Catholic Church disapproval will do nothing to dent the Berlusconi political machine.

For example, in reference to the recent Mills court judgment which saw his British former lawyer David Mills condemned for having committed perjury on behalf of the Berlusconi empire, Berlusconi this week offered his standard defence.

Speaking to the retailers confederation, Confesercenti, he said there were “subversive clots” within the judiciary. An immediate opinion poll found that more than 65 per cent agreed with him.

Questions raised over Berlusconi’s link with Neapolitan model (18)

Mr Berlusconi has been caught out telling numerous lies about the relationship and refuses to explain them. And with important elections pending, his popularity, at an all-time high only six weeks ago, may be eroding.

The media cannot be accused of muck-raking on the issue because it was Mr Berlusconi himself who drew attention to the relationship in Tuesday when he took advantage of a trip to Naples to drop in on Noemi’s 18th birthday party.

There he posed for photographs and presented the pretty young blonde with a gold and diamond pendant worth NZ$14,500. This unremarkable event was immortalised in a short news story the next day in La Repubblica.

And there it would have ended, except that within four days it provided Mr Berlusconi’s second wife, Veronica, with the casus belli for a divorce.

Her husband, she said in a press release, was “consorting with minors”; he was “not well”, she was worried about him, but in the meantime, after nearly 30 years together, she was in no doubt that the marriage was over.

Suddenly that innocuous-seeming social event assumed mysterious and sinister overtones. Noemi, it was learned, called Mr Berlusconi “papi”, Italian for “daddy”. He seemed on remarkably familiar terms with the girl.

Pushed into a corner by Veronica, who opens her mouth about once every two years but with devastating effect, Berlusconi went on Porta-a-Porta, a late-night political chat show hosted by his most unctuous TV courtier and explained that Noemi’s father Elio Letizia was an old political contact from his days when he was connected to Bettino Craxi and the Socialist party: Berlusconi needed to see him on urgent European election business.

But soon afterwards Bobo Craxi, son of the late Bettino, popped up and said he had never heard of Noemi’s father. Likewise Mr Berlusconi’s unlikely claim about “election business” failed to pan out, and some weeks later was denied by Letizia himself.

The personal was personal no more: something about that birthday party, and Mr Berlusconi’s presence at it, had tipped the long-suffering Veronica over the edge. One reason for her anger, as she explained in a bitter email to the Ansa news agency, was the fact that he had failed to turn up to the coming-of-age parties of any of the their own children, “even though he was invited”.

Could a teenage girl topple Berlusconi?