Italy's center-left backed former Prime Minister Romano Prodi as candidate for president on Friday, April 19, setting up a battle with the center-right alliance of Silvio Berlusconi and increasing the likelihood of a snap election within week, Reuters reported.

The election of the next head of state to succeed President Giorgio Napolitano, whose term ends on May 15, is crucial to ending a two-month stalemate since a parliamentary poll in February which left no party able to form a government.

The selection of Prodi represents a stark turnaround for center-left leader Pier Luigi Bersani, whose attempt to impose 80-year-old former Senate speaker Franco Marini on the party after an agreement with Berlusconi failed on Thursday.

Democratic Party (PD) president Rosy Bindi said after the meeting that the choice of Prodi had removed the threat of a split and the party was united behind him. "The party expressed itself unanimously," she told reporters.

However the choice was immediately rejected by former premier Berlusconi's camp, which said it was likely to prevent any government being formed and lead to early elections.

"If the PD is answering us in this fashion, it's not us choosing elections, it's the PD that's adopting a position of total opposition to us," said Fabrizio Cicchito, a close ally of Berlusconi.

Marini fell far short of the two thirds majority required for election after dozens of center-left electors cast blank ballots in the first round of voting, effectively sinking his candidacy and forcing the party to look for a new candidate.

The president is elected by a joint sitting of the two houses of parliament joined by 58 regional delegates in a complex process requiring several rounds of voting.

The first three rounds of voting require a two-thirds majority but in subsequent rounds only a simple majority is necessary, meaning that Prodi could pass with the support of centrists or the anti-establishment 5-Star Movement.