Rome (CNN) -- Italian Prime Minister Silvio Berlusconi had sex 13 times with underage dancer Karima el Mahroug, nicknamed "Ruby the heart-stealer," prosecutors say in legal papers seen by CNN.
The allegations come in legal papers accusing three of Berlusconi's associates with aiding and abetting prostitution.
The three are accused of selecting up to 33 young women, including 17-year-old el Mahroug, to attend parties at Berlusconi's house, where the women would perform erotic dances before the prime minister chose one to spend the night with.
Berlusconi and el Mahroug both deny that they ever had sex.
He laughed off the allegations Wednesday, according to a leading Italian newspaper.
"I'm 75, and even if I'm a bit mischievous, 33 girls in two months is too much even for someone who is 30," the daily La Repubblica quoted him as saying.
He is being tried separately from the other three on charges of having sex with an underage prostitute and abusing his position as prime minister to help el Mahroug after she was arrested for theft. He has called both allegations an attempt to smear him.
The Milan prosecution papers, filed Tuesday, name regional lawmaker Nicole Minetti, Berlusconi employee Emilio Fede and talent scout Dario "Lele" Mora.
In an exclusive CNN interview last month, Minetti denied ever procuring prostitutes for Berlusconi and said the media had misrepresented the parties at his villa.
A lawyer for Berlusconi has asked Rome prosecutors to investigate an alleged attempt to falsify el Mahroug's birth records, he said Tuesday.
The alleged falsification appears to be an attempt to make her appear not to have been a minor when Berlusconi allegedly paid to have sex with her.
Rome prosecutors did not immediately respond to questions about the case.
The Italian daily Il Fatto Quotidiano reported last week that someone had tried to falsify the birth records.
Berlusconi's lawyer, Nicolo Ghedini, said in a statement that he asked Rome prosecutors to investigate the allegation that two Italian "emissaries" had visited a birth-registry official in Morocco, where el Mahroug was born, and offered bribes to make the young dancer seem older.
Her age is significant because it is illegal to have sex with a prostitute in Italy only if she is a minor.
Ghedini suggested in the statement that he requested the investigation to establish that Berlusconi had nothing to do with any attempt to change birth records.
"We are dealing with something that tries to surreptitiously damage gravely Prime Minister Berlusconi, who is completely estranged to any eventual illegal action," the prime minister's lawyer said in a statement.
Berlusconi has been tried on at least 17 charges since his first term as prime minister began in 1994, but none of the cases has resulted in lasting convictions. Several were overturned on appeal.
CNN's Livia Borghese contributed to this report.