Italian prosecutors have filed charges against former prime minister Silvio Berlusconi for allegedly bribing a left-wing senator $3.9m to join his party.

The announcement from the prosecutor's office on Thursday in Naples comes a day after a Milan court upheld Berlusconi's conviction in a separate case of tax fraud linked to his Mediaset business empire.

Wednesday's ruling confirmed a sentence of four years in prison against Berlusconi and a three-year amnesty, leaving a one-year prison sentence which is suspended pending an appeal.

Prosecutors on Thursday alleged the media baron paid off senator Sergio De Gregorio after elections in 2006 won by a centre-left coalition led by Romano Prodi by just a handful of votes.

A few months later, De Gregorio crossed the aisle and joined the Berlusconi opposition in a move that helped bring down Prodi in 2008.

The subsequent elections were won handily by Berlusconi.The investigation is based in Naples since that was De Gregorio's seat.

There will now have to be an indictment hearing to decide whether or not we to implement the charges and begin a new trial against Berlusconi.

The scandal-tainted Berlusconi, 76, has been a defendant in multiple court cases in his two decades in politics and has two convictions.

He is also a defendant in a trial in which he is accused of paying for sex with an underage 17-year-old prostitute while he was prime minister and for abusing the powers of his office.

Source: Agencies