Antonio Ingroia called Marcello Dell'Utri the mafia's ambassador to Berlusconi's Forza Italia party and said the former aide regularly received input from mob leaders.
"Dell'Utri worked with input from Cosa Nostra, he was the ambassador of Cosa Nostra," Ingroia told La7 television.
Ingroia, the leader of the new Civil Revolution party, contended Berlusconi and Dell'Utri formed Forza Italia with the consent of the mafia, Italy's ANSA news agency said.
Dell'Utri faces a retrial on charges of assisting the mafia. He was granted a new trial on the grounds his legal rights had been violated.