Silvio Berlusconi. File picture: Yara Nardi

Rome - A new trial was due to start in Naples on Tuesday against Italy's scandal-prone former premier Silvio Berlusconi, on charges of bribing an opposition lawmaker to switch over to his side eight years ago.

In 2006, Sergio De Gregorio was elected with the centre-left coalition of Romano Prodi, but defected soon after. He was re-elected for Berlusconi's conservatives in 2008, and remained a senator until last year.

As his parliamentary immunity expired, he confessed to prosecutors that Berlusconi bribed him with three million euros.

In October, he was sentenced to a 20-month-jail term, under a plea bargain agreement.

Lawyers for Berlusconi, who is a billionaire media mogul as well as a politician, have defended the payment to De Gregorio as legitimate financial aid for the small party the senator founded after his left-right switch.

The prosecution sees the alleged bribe as part of a plan to bring down Prodi, who won in 2006 by a wafer-thin margin. He survived in office until 2008, when his coalition finally collapsed, paving the way for early elections and Berlusconi's return to power.

Tuesday's opening hearing was expected to focus on procedural issues - namely, a change in the composition of the court.

One judge asked to be excused because she is the wife of another judge involved in a different Berlusconi case.

Despite his legal woes - he was ejected from parliament last year following a tax fraud conviction, and he is appealing a guilty ruling for soliciting sex from a minor and abusing his position to cover the affair - Berlusconi remains a dominant political figure in Italy.

He leads the opposition Forza Italia party, and is co-operating with his centre-left rival, Florence Mayor Matteo Renzi, on drafting a new electoral law and other key reforms. Their pact may falter if embarrassing details emerge in court.

However, observers say there will not be enough time to end the trial and appeals proceedings before the statute of limitations is due to expire in late 2015, meaning that the entire case is likely to be dropped, saving Berlusconi from the risk of another conviction.