The claims are made in a book to be published by Elisa Alloro, 32, a former employee of Mr Berlusconi's television network.
The Prime Minister has become embroiled in a sleaze row after deciding to put forward showgirls as MEP candidates ahead of the European elections.
The book, called We, Silvio's Girls, is written as an extended letter to Veronica Lario, Mr Berlusconi's second wife, who has been openly critical of her husband.
Ms Lario said that she wanted a divorce after it was revealed that her husband had been to aspiring model Noemi Letizia's 18th birthday party and called him Papi.
"The Prime Minister is a mine of wisdom," Ms Alloro said. "I have always considered every moment I have spent with him a gift from God."
She added that she "vaguely remembered" Ms Letizia at the villa at the party, which the teenager is said to have attended with a number of female friends, The Times reported.
Ms Alloro said that the term Papi had been used by many of his women friends for some time and was a code name in case their conversations were overheard.
It comes after more details emerged of parties at Villa Certosa, Mr Berlusconi's Sardinian villa, including photographs showing topless women.
According to the Italian press, some of the pictures of the parties show topless women embracing under a shower beside a swimming pool.
The reports said that Mr Berlusconi was clothed in all the photographs, some of which showed him driving girls round his estate in a golf buggy while armed guards stood on the perimeter wall.
