An ally of Italian PM Silvio Berlusconi during the years when he was a businessman acted as the leader's "liaison" with the Mafia, a Sicilian appeals court ruling has said.
In June, the court upheld a conviction for mafia links against Marcello Dell'Utri, a senator for Mr Berlusconi's People of Freedom Party.
The judges explained on Friday the reasons for Dell'Utri's conviction.
Both men have previously denied any accusations of links with the Mafia.
According to the court's 600-page document, Dell'Utri "carried out... an activity of mediation as a link between the mafia association Cosa Nostra, represented by its most influential representative at the time, Stefano Bontate, and Silvio Berlusconi".
Mafia bosses in turn offered "protection" for Berlusconi, it added.
Dell'Utri "consciously chose to mediate between the interests of the mafia and the business interests of Berlusconi, allowing the criminal group to profit, promoting the consolidation and strengthening of Cosa Nostra".
The judges also said that, before entering politics, Mr Berlusconi paid "enormous sums of money" to Cosa Nostra for protection and later for installing his TV channel's relay stations in Sicily, following a request made by Dell'Utri.
Dell'Utri, who was first sentenced in 2004 and is now awaiting a final ruling from Italy's highest appeal court, responded to the publication, saying: "There's nothing new. It's a subject that has been gone over many times."
It is not clear whether the ruling has any implications for Mr Berlusconi.