ROME (Reuters) - The story of Silvio Berlusconi's friendship with an 18-year-old woman has transfixed Italy and forced the prime minister onto the defensive as he leads his party into European elections and prepares to host a G8 summit.
Italy's opposition, media and even the Church have been hammering him to clear up the nature of his relationship with Noemi Letizia, an aspiring model who wants to break into show business.
Opponents sense a rare chance to land a blow on Berlusconi, who dominates the political landscape and remains popular despite the impact of the global crisis -- Italy's economy shrank 1 percent in 2008, the worst contraction since 1975.
"A politician has a duty to respond and a duty to tell the truth," Dario Franceschini, head of the opposition Democratic Party which trails Berlusconi's conservatives by about 15 percentage points in polls, told a rally at the weekend.
Letizia, whose 18th birthday party Berlusconi attended in Naples last month, is at the center of a messy divorce suit by his wife Veronica Lario.
Berlusconi, accused by Lario of "frequenting minors," insists nothing improper happened between him and the teenager who refers to him as "papi" (Daddy).
He says the opposition is trying to make political hay of the story to discredit him ahead of the June 6-7 European elections and the July 8-10 G8 meeting when he will host world leaders including U.S. President Barack Obama.
But the saga has become so politically explosive that Berlusconi has vowed to clear his name with a formal address to parliament, something he rarely does.
"He is rattled because he feels he is not in control. He obviously doesn't want to have Noemi on the front pages when he is meeting Obama," said James Walston, political science professor at the American University of Rome.
La Repubblica, Italy's second largest mainstream daily, has been leading the charge against Berlusconi and breaking most of the stories, provoking the wrath of the premier and his aides.
On Tuesday, the left-leaning paper ran an investigative story whose headline accused Berlusconi of offering the country "a rosary of lies." Every day since May 14 it has published 10 questions it says he should answer to clear up the controversy.
Berlusconi has come back fighting in a rare spate of interviews. He told CNN International it was "shameful" that the private matter of his divorce was being used for political attack. He added: "There is nothing, nothing of nothing, that is even remotely negative" about the relationship with Letizia.
"Now they accuse me of lying ... and I will respond," he said. No date for the parliamentary speech has been set.
The furor began last month when Berlusconi's wife Lario expressed her anger after he attended Letizia's 18th birthday party and gave her a 6,000 euro ($8,382) gold necklace.
Lario, who has also criticized what she says is a bevy of attractive women chosen to run for office in her husband's party, said he had never attended the 18th birthday parties of their own children.
Since then, newspapers have wanted to know, among other things, if Berlusconi ever was with Letizia when she was not accompanied by her parents and was still a minor.
Berlusconi has said he has known the family for years.
But barely an hour passes without Italian news agencies running dispatches about the case. More than 100 were transmitted on Monday alone -- and that was a slow Noemi day.
La Repubblica has quoted Letizia's former boyfriend as saying she was among overnight guests at a New Year's party at one of Berlusconi's villas on Sardinia. On Monday Corriere della Sera, a rival paper, quoted the prime minister as telling his aides that she had indeed been among the many guests "but I don't understand why this is a scandal."
Italy's Catholic Church, which so far has had a good relationship with Berlusconi because of his stands on bio-ethical issues, has questioned his lifestyle.
"Scandals in one's private life undermine the personal credibility of a political leader," Archbishop Alessandro Plotti, the former vice-president of the Italian Bishops Conference, said in the Turin newspaper La Stampa.
Editing by Mark Trevelyan