ROME - Italian Prime Minister Silvio Berlusconi and his fiercest rival Gianfranco Fini are girding for a final showdown which political analysts say is likely to lead to a government collapse in a matter of weeks.

Fini, the once-loyal lower house speaker who bitterly split with Berlusconi, is expected to withdraw a minister, a deputy minister and two undersecretaries from the government on Monday.

While that will not in itself sink the government, it will be another nail in the coffin for Berlusconi, further escalating a political crisis that reached boiling point in the last week.

Seeking to regain the political initiative, Berlusconi announced late on Saturday that he would call a do-or-die confidence vote in both chambers of parliament after the 2011 budget is passed -- probably around mid-December.

While he still enjoys a narrow majority in the Senate, he can no longer count on one in the lower house after Fini's defection.

"Berlusconi can limp along for a month or so, but the balance of power is no longer in his favour," said Maurizio Pessato, head of polling firm SWG.

"There has been a shift in public opinion, and with the economic crisis he has also lost the backing of powerful business lobbies. This time, it looks like the tide is turning against him."

A defeat in parliament would force him to resign, opening up several scenarios, including an early election next year.

Berlusconi's last remaining coalition ally, the Northern League, says he could try to form another government.

But Fini has rejected that option, saying Berlusconi must step down to pave the way for a new centre-right coalition.

Faced with a full-blown political crisis, President Giorgio Napolitano could appoint an interim government to run business until new elections, like the 1995 administration headed by former Finance Minister Lamberto Dini that followed the collapse of Berlusconi's first government.

Berlusconi says that if Fini brings him down the only alternative is snap polls in early 2011, two years ahead of schedule. His gamble is that voters will return him to office.

"Don't read the newspapers. Voters still exist and 60 percent of them are with me," Berlusconi told a rally on Sunday.

Opinion polls don't back him up. A survey of voters' intentions in Corriere della Sera daily gave a coalition between Berlusconi's People of Freedom (PDL) party and the Nothern League only a marginal lead over the centre-left opposition.

Fini's new movement stood at around 8 percent with the centrist UDC party at 5.8 percent -- making a "Third Way" bloc potentially determinant for any new majority.

An ex-fascist turned mainstream conservative, Fini co-founded Berlusconi's PDL party in 2008. But after months of acrimonious exchanges, Berlusconi expelled him from the PDL in July. The break-up deprived Berlusconi of a guaranteed majority in the lower house, virtually paralysing the executive.

END OF AN ERA?

The crisis reached new heights last month when it emerged that Berlusconi had called a Milan police station in May to ask for the release of a 17-year old nightclub dancer held for theft. The girl, known by the stage name of Ruby Rubacuori (Heart-Stealer) says she attended parties at his villa.

Berlusconi has denied any interference with the justice system in the case. But the latest scandal emboldened his critics, who say his raucous lifestyle is distracting him from running a country only slowly emerging from its worst post-war recession and with a huge public debt mountain.

With media printing pictures of the scantily-clad Ruby next to those of garbage piles in Naples, Pompeii's collapsing ruins, and floods in the Veneto region, Berlusconi has found it harder than usual to dodge the latest controversy.

The 74-year old billionaire, who won a landslide election in 2008 thanks to his personal charisma and ability to capture the mood of voters, has bounced back before, surviving a string of sex scandals and corruption accusations.

But while it may be premature to write him off, most analysts say the countdown to the end of the Berlusconi era, even among his allies, has begun.

"In this rainy Italian autumn, it is not just a majority or a government that are approaching the end. It is the journey of a lonely man that is nearing its conclusion," wrote Corriere della Sera in an editorial on Sunday.