Prime Minister Silvio Berlusconi is buying votes in parliament ahead of a crucial confidence vote next week, according to allegations by the centre-left opposition being investigated by Italian prosecutors on Friday.

The billionaire media tycoon faces confidence votes in both houses of parliament on December 14th. A defeat in either chamber would force him to resign and possibly lead to early elections next spring.

He has repeatedly said he is confident of winning even in the lower house, where he no longer has a guaranteed majority since splitting with his former ally Gianfranco Fini.

Centre-left politicians and Fini have accused Berlusconi and his People of Freedom party of bribing lawmakers in their ranks to buy their support after some opposition MPs said they would back Berlusconi or abstain in the vote.

Berlusconi, accustomed to scandal, has rejected the accusations, saying his political enemies had miscalculated their own strength, wrongly writing him off ahead of time.

"I don''t think that there will be that many traitors, we will have the majority," he said late on Thursday.

Antonio Di Pietro, leader of the small Italy of Values (IDV) centre-left party and one of Berlusconi''s most vocal critics, filed a complaint with Rome prosecutors on Friday, leading to the opening of an official probe.

"We are witnessing corruption in parliament," said Luigi De Magistris, also from the IDV party. "Money, favours and other stuff is being offered; this is an issue for the judiciary."