Prosecutors say Italian PM Silvio Berlusconi paid for se.x 13 times with Karima El Mahroug, also known as Ruby, when she was still 17, reports say.
They have filed a document requesting indictments against three of the prime minister's associates for allegedly soliciting prostitutes for him.
They allege that 33 women took part in erotic parties at his villa near Milan.
Mr Berlusconi, who faces trial on 6 April, has condemned the latest allegations as "without sense".
He denies having s.ex with an under-age prostitute and abuse of power. Ms Mahroug denies having s.ex with the Italian leader.
Prostitution is not a crime in Italy, but paying for or helping arrange s.ex with a prostitute under the age of 18 is an offence.
Karima El Mahroug also known as Ruby