Italy's divided parliament has begun voting for a new president with the centre left alliance led by Pier Luigi Bersani deeply split over the choice of former Senate speaker Franco Marini as their candidate.

Bersani nominated Marini, a prominent Catholic and former head of the moderate CISL union, in order to forge a broad accord on the presidency with centre-right leader Silvio Berlusconi and the small centrist grouping of caretaker Prime Minister Mario Monti.

But the choice provoked fury in Bersani's Democratic Party (PD) and an open revolt by his rival, Matteo Renzi, the 38-year-old mayor of Florence.

He said a vote for Marini would be a "disservice to the country".

The vote on Thursday for a successor to President Giorgio Napolitano, whose term ends on May 15, is a crucial step towards resolving the stalemate since an inconclusive election in February which left no party with enough support to form a government.

Bersani has repeatedly refused to agree to Berlusconi's demands that they form a broad coalition government together.

But it is widely believed he wants to parley an agreement on the presidency with centre-right willingness to support a minority centre-left government.

Renzi described the 80-year-old Marini as "a candidate from the last century" who had no charisma or international standing, adding that he was only chosen because he was acceptable to Berlusconi.

Uncertainty

As many as 90 of the 430 PD parliamentarians voted against Marini at a party meeting on Wednesday night, meaning that even with the votes of Berlusconi and the centrists, it is uncertain whether he can win the two-thirds majority required for victory in the first three votes in parliament.

"The PD is in fragments, it doesn't exist anymore," Renato Brunetta, the parliamentary leader of Berlusconi's People of Freedom (PDL) party, told Canale 5 television.

Even by the tangled standards of Italian politics the situation is complicated, but until the new president is elected, the paralysis that has hobbled attempts to form a government for more than 50 days since the election will continue.

On Wednesday, the anti-establishment 5-Star Movement led by former comic Beppe Grillo named its candidate as Stefano Rodota, a left-wing academic who is anathema to Berlusconi but who many in the centre-left would be prepared to support.

Bersani's leftist allies in the Left Ecology and Freedom (SEL) party said Marini was unacceptable and that their 46 parliamentarians would vote for Rodota.

Ceremonial figure

A joint sitting of the two houses of parliament, joined by 58 regional delegates, began voting on Thursday at 10 am (0800 GMT), but no result may be reached before the weekend if Marini does not secure sufficient backing.

Two rounds of voting will be held every day, with a two-thirds majority or 672 of the 1,007 electors needed in the first three rounds.

The PD, which controls the lower house, has the largest number of electors with 430, ahead of the PDL with 211 and the 5-Star Movement with 162.

If no candidate succeeds in the initial rounds, the required threshold drops to a simple majority from the fourth round. But it is unclear whether Marini would remain in the race or drop out if he failed to secure the two-thirds majority.

That could lead to the PD abandoning hopes of a deal with the centre-right and going for a candidate like former Prime Minister Romano Prodi, one of Berlusconi's oldest political enemies.

Such an outcome is widely seen as likely to lead to an election within months.

The head of state is a largely ceremonial figure but has a number of vital political functions, as Napolitano demonstrated in 2011 when he put Mario Monti at the head of a government of technocrats to replace the scandal-plagued Berlusconi.

It will be up to the new president to end the political deadlock left by the election, either by persuading the parties to come to an accord that would allow a government to be formed or by dissolving parliament and calling a new national vote.