The debate raging in Italy about Silvio Berlusconi's alleged taste for teenage girls and prostitutes has taken a surreal turn, after the country's top porn star weighed in with high praise for the prime minister. "The truth is," said Rocco Siffredi, "that Italians are proud of someone like Berlusconi who is 74, loves sex and has a good sex life – and I don't just mean working-class Italians."

Siffredi, 46, was named by an adult film industry listing as one of the world's 50 top porn stars and is now a celebrity, appearing on Italian chat shows. He told the Observer that, if Berlusconi can avoid jail for allegedly paying a teenager for sex and pressuring the police into covering it up, he will be forgiven by Italian voters. "In any other country he would have been forced out years ago, but here he gets away with it," said Siffredi.

Following Berlusconi's refusal to submit to questioning, Milan magistrates will now request he stand trial before the summer without a pre-trial hearing. His lawyers are expected to try to halt the trial by arguing that it should be handled by a special tribunal for politicians.

Two years after Berlusconi's wife left him over his friendship with teenager Noemi Letizia, and a prostitute, Patrizia D'Addario, claimed to have slept with him, Berlusconi is again facing criticism after accusations that he had sex with pole-dancing prostitutes at parties at his mansion near Milan. One guest, Nadia Macrì, described Berlusconi lying on a bed calling out "Next" as women queued to have sex with him.

Berlusconi's troubles deepened tonight when for the first time one of the women named in the prostitution investigation admitted that she had sex with him. Maria Ester Garcia Polanco, 25, denied that she did so in exchange for money but her admission makes her the first of the so-called "bunga bunga" girls – a group of 14 women whom Milan prosecutors allege played striptease games and had sex with the prime minister – to state publicly that she slept with him.

Polanco told La Repubblica she did so out of gratitude because he had paid for urgent medical treatment for her five-year-old daughter and had helped her find work as a television showgirl.

Prosecutors are also examining allegations that Berlusconi paid to sleep with Moroccan nightclub dancer Karima el-Mahroug when she was 17. It has also been alleged that Berlusconi pressured police to free her when she was later arrested on suspicion of theft. On Thursday, in a statement clearly directed at Berlusconi, the Vatican demanded "a more robust morality and sense of legality" from Italy's rulers.

The fallout from the continuing revelations has created the most serious test of Berlusconi's premiership. Opposition leader Pierluigi Bersani has vowed to collect 10 million signatures calling for Berlusconi to step down. Meanwhile, the head of the residents' association at a plush Milan apartment complex last week evicted a dozen or so "models" allegedly housed there by Berlusconi.

Beppe Severgnini, a columnist, warned it was too soon to write off Berlusconi. "All Italians have an affinity with him," he said. "He is a walking absolution for our sins – and we have been living in the valley of complicity for a long time."

Half of the Italians interviewed for a poll as the scandal broke said the latest headlines would not affect Berlusconi and could even push up his ratings. Scepticism was not hard to find on Rome's streets this weekend. "He should be less obvious about it, but he is free to do what he wants – look at Bill Clinton," said Francesco Nobili, 33, a personal trainer. Salvatore, a blacksmith, said: "I would give my daughter a slap if she went to a party at Berlusconi's, but I still vote for him since I approve of his politics."

Berlusconi's fightback got under way with video messages claiming that his parties were innocent affairs and that magistrates had operated outside the law by wire-tapping his guests. Supporters were drafted onto chat shows to pour doubt on el-Mahroug's claims that Berlusconi knew she was under age.

Siffredi, who crossed paths with Mahroug on a talk show on Tuesday, said he suspected that Berlusconi would not worry too much about a girl's real age if she looked over 20. He described Berlusconi as a sex addict. "I have sex one half of a half of a half of the times he does," he said in a Facebook video. By mixing power with sex, he added, "by doing to whoever you want what you want", Berlusconi was living out his own fantasies.

That approach, he added, would not lose him votes in Italy. "Just as my appeal is based on being a family man off the set, Berlusconi understands that being a family man is important, as is staying on the right side of the church," he said.

"Berlusconi and I have a lot in common," Siffredi added. "I have been reliably informed that he once said 'Siffredi and I both have the same problem: priapism'."