ROME (AFP) - An Italian judge on Wednesday ordered Silvio Berlusconi to stand trial for allegedly bribing a senator to join his party's ranks, in the latest legal woe for the former prime minister.

Berlusconi was formally charged along with his former associate, Valter Lavitola, who is accused of acting as an intermediary in the 3.0-million euro (S$5.1 million) bribe, lawyers said.

The senator himself, Sergio De Gregorio, helped investigators and was granted a 20-month sentence under a plea bargain by the hearing in Naples.

The trial is due to start on February 11, 2014.

"I urge Silvio Berlusconi to leave the political scene, which would free Italy from a lot of dirt," De Gregorio told news channel SkyTG24.

Lavitola told the hearing that even if he had handled the money as alleged "there is no proof that I could have known that it was money for a bribe, I would have been simply a conduit".

But De Gregorio said: "He was the intermediary for Berlusconi in bringing me the money."

Lavitola was first placed under investigation in 2011 and fled the country for South America.

He returned after eight months on the run in April 2012 and was arrested at the airport.

The De Gregorio case goes back to elections in 2006 which were 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 next elections were won handily by Berlusconi.

"I now believe I behaved in an absolutely reprehensible way and was aiming to bring down the Prodi government as part of a sort of holy war being waged by Berlusconi," De Gregorio said.

"I have said sorry like no one does in Italy."

The investigation has been handled by prosecutors in the southern city of Naples since that was De Gregorio's seat and the trial will also be there.

Supporters of the 77-year-old Berlusconi immediately rallied around the three-time former prime minister, accusing prosecutors of bias.

"I can't believe it," said Daniele Capezzone, a lawmaker from Berlusconi's People of Freedom party.

"I think Italians fully understand that what has been happening in the past few years is an attack against a political leader who was freely and democratically chosen by millions," he said.

Another ally, senator Renato Schifani, said: "The encirclement of Berlusconi is continuing with a few extra knocks. The aim will not be achieved."

Berlusconi has often been accused by opponents of buying votes, but this is the first time he has officially been charged of corrupting a politician.

Berlusconi was convicted definitively of tax fraud in August in a ruling that could end up ejecting him from a parliamentary seat for the first time since he burst onto the political scene in 1994.

He also faces 12 months of community service in the case and being barred from running in parliamentary elections for the next six years.

His lawyers have lodged an appeal with the European Court of Human Rights in Strasbourg over the new law, introduced last year with votes from Berlusconi's own new party, to exclude convicted criminals from parliament and elections.

Berlusconi is also appealing convictions for having sex with an underage prostitute and abuse of prime ministerial powers, as well as for leaking a confidential police wiretap in one of his newspapers to damage a political opponent.

The tycoon regularly protests his innocence and says he is the victim of vindictive left-wing judges who want to eliminate him politically.