Italy's prime minister is accused of paying an underage prostitute. But a lawyer for Karima El Mahroug said the alleged escort would not be attaching herself to the case – denying a move that may have allowed her to seek damages from the 74-year-old billionaire.

The prosecution maintains there is evidence to show that Mahroug, then 17, stayed at Berlusconi's villa at Arcore, outside Milan, on several occasions between February and May 2010 and that wiretaps, said to be of conversations between the young Moroccan and others, indicate she had sex with the prime minister while there.

But her lawyer, Paola Boccardi, told the court: "She did not feel it was right to make herself a party to the case because she feels she did not suffer any damage from having gone a few times to Arcore, nor from having known the prime minister."

Berlusconi, who denies all wrongdoing, is also charged with abusing his position to cover up his alleged offence.

Last May, after Mahroug was detained on suspicion of theft, he telephoned a Milan police station, claiming she was the granddaughter of the then Egyptian president, Hosni Mubarak. Instead of being returned to care, the runaway was entrusted to an associate of Berlusconi who put her in a flat with a Brazilian prostitute.

The prosecution has cited a junior Milan police officer, Giorgia Iafrate, as the victim of the alleged offence of abuse of office. But Iafrate's lawyer said she too had decided against making herself a party to the case.

The trial, which was adjourned until 31 May promises to be the most sensational of the many in which Berlusconi has been a defendant. Among those called to testify is the US actor George Clooney, who has denied Mahroug's claim that he was a fellow guest at an Arcore dinner.

According to the prosecution, these were often the prelude to so-calledbunga bunga sessions in which the women guests performed erotic dances, before Berlusconi chose one or more of them with whom to spend the night. Investigators gathered evidence of substantial payments to the prime minister's young female guests, including Mahroug.

But the Moroccan's lawyer said that making herself a party to the case would have been at odds with her position that she had never had sex with the prime minister. He added: "The other reason for her decision is that Karima has always maintained she was never a prostitute, while this trial takes it for granted that she made herself available in return for payment."

Mahroug has been called as a witness in the case by both the defence and the prosecution. She has several times given apparently contradictory accounts of her relationship with Berlusconi, and until now it was unclear what position she would take at his trial.