Silvio Berlusconi and Ruby 'exchanged 53 messages in three months'

The latest details came as Mr Berlusconi's trial started and was adjourned after only eight minutes.

The prosecution papers, as reported by La Repubblica newspaper, allege that the pair exchanged messages between February and May last year, when she was only 17, the last of which came a month before she was arrested.

Mr Berlusconi denies any wrongdoing and the Moroccan-born teenager, Karima El Mahroug, also known by the stage name Ruby the Heart Stealer, insists she never slept with him.

At the time of the alleged offences she was 17 – a year beneath the legal age of prostitution under Italian law.

She was just one of dozens of young models, showgirls and aspiring actresses who reportedly entertained the prime minister with strip teases and pole dances at his residence outside Milan, in return for cash, jewellery and help with their careers.

The trial was adjourned until May 31, to give the judges enough time to consider a request by a women's organisation, Arcidonna, to be admitted as a civil party to the case on the basis that Mr Berlusconi's behaviour has "offended the dignity" of all Italian women.

Paola Boccardi, Miss El Mahroug's lawyer, said her client would not apply to become a civil complainant in the trial, a move that would have allowed her to seek financial damages from Mr Berlusconi if he is found guilty.

"Karima does not believe herself to have suffered any damage from having been present at Arcore, or having visited the premier," she said.

The trial is expected to take months, if not years, because of the grindingly slow pace at which the Italian court system works and the sheer volume of material to be scrutinised.

Prosecutors have filed 20,000 pages of evidence and have requested that 132 people be called as witnesses.

Mr Berlusconi's lawyers have presented a witness list of 78 people, including the actor George Clooney, whom Miss El Mahroug claims was at one of the prime minister's dinners – an account the Hollywood star denies.

Rival groups of protesters hurled insults at each other outside the court building, which was ringed by paramilitary police equipped with riot shields and helmets.

"It's all false and it is an outrage that the investigators have spent millions of euros wiretapping the phones of the prime minister and his friends," said Mariagrazia Piracci, 53, who sells advertising space. "He is absolutely innocent."

But anti-Berlusconi demonstrators carried banners claiming that Mr Berlusconi's wealth and power had put democracy at risk.

On the eve of the hearing, Mr Berlusconi was able to secure the approval, through his coalition's parliamentary majority, of a motion that challenges the Milan court's jurisdiction and argues that the case should instead be heard by a special tribunal of ministers.

The approval of the motion will not stop the trial going ahead for the time being but is due to be considered by Italy's Constitutional Court, which will decide in coming weeks whether to transfer the trial to the tribunal in Rome, which deals with offences committed by MPs.