Silvio Berlusconi has condemned outgoing Prime Minister Mario Monti's plan to lead a centrist alliance in Italy's election in February.

Mr Berlusconi accused him of a plot to favour the left, but centrist leaders denied any secret accord.

Monti, who replaced Berlusconi as prime minister last year as Italy scrambled to avert a financial crisis, said he wanted to unite a broad coalition of factions around a reform agenda aimed at easing the country's economic woes.

Monti ended weeks of speculation when he confirmed his bid for a second term, pitting him against the centre-left Democratic Party and Berlusconi's centre-right People of Freedom party in a three-way contest.

Speaking to reporters at Milan Central railway station, Berlusconi said Monti wanted to help the left secure power after the February election so he could continue his austerity agenda of tax hikes and spending cuts.

"This grouping has been formed to favour the left - also the harmony with the left's programme they have celebrated heads in this direction," he said, after earlier describing Monti as "the spare wheel" of the PD in an interview with Vista TV.

The 76-year-old billionaire, who caught the train from Rome with his new 27-year-old partner, Francesca Pascale, said he did not believe Italian voters would "fall into the trap" which he said was aimed at stealing votes from the centre right.

But Pier Ferdinando Casini, head of Italy's oldest and largest centrist party, the UDC, which is cooperating with Monti, strongly denied the accusations.

"Our initiative was not born with the support of the PD. It has not been started with a predetermined alliance ... until election day, what's important is aiming for the majority."

The daily Corriere della Sera has reported that Mr Berlusconi has agreed to pay his estranged wife Veronica Lario €100,000 a day as part of a divorce settlement.

The newspaper said the €36m-a-year settlement, reached after three years of negotiations, was filed with a court in Milan around Christmas.

No comment was immediately available from the court or from lawyers for either Mr Berlusconi or Ms Lario following email and telephone requests for confirmation.

Mr Berlusconi did not make any comments to the media as he boarded the train in Rome. 

The former premier Berlusconi, who just over a year ago stepped down as prime minister, recently decided to return to frontline politics to lead the centre-right campaign ahead of the 24-25 February elections.

He has launched a media blitz against Mario Monti with a series of angry attacks against his "Germano-centric" austerity policies.