Prodi fell well short of the absolute majority of 504 that would have been needed for him to win the presidency in the Friday vote.

Although he was endorsed by all center-left parties, and should have been able to count on almost 500 votes from them, the ex-premier only secured 395.

Some 100 center-left politicians appeared to have refused orders from their own parties to vote for Prodi. Because voting takes place in secret, electors are not obliged to toe the party line.

"It is clear that we are going nowhere with these numbers," Francesco Boccia of the Democratic Party (PD) told the broadcaster SkyTG24r.

Another leading PD figure, Florence Mayor Matteo Renzi said the vote had effectively ended Prodi's candidacy. The 73-year-old Prodi currently serves as the UN envoy for the Sahel region of Africa.

Right-wing leader Silvio Berlusconi had also strongly opposed the nomination of former arch-rival Prodi, who twice defeated him for the premiership.

Setback for Bersani

The outcome is a further blow to center-left leader Pier Luigi Bersani, who met with revolt from leftist politicians within his coalition at his initial efforts to have former Senator Franco Marini installed as president.

Marini was ditched as a candidate after his failure to win the third round of voting in Italy's complex system of choosing a president, which would have required a two-thirds majority.

Bersani then told his party members to vote for Prodi in the following round but they disobeyed him. Afterwards Bersani announced that he would resign as soon as a new head of state was elected. "One in four of us was a traitor," he said after the vote. "For me, it is unacceptable."

In the fourth round of voting, Prodi would only have needed more than half of the 1,007 regional and national lawmakers to cast their vote for him.

A fifth round of voting is scheduled for Saturday.

The election of a new president has been seen as an important step towards ending the political deadlock that has paralyzed the country's legislature since inconclusive parliamentary elections in February.

Although the president has no political roll, he would be able to dissolve parliament and call elections. Outgoing President Giorgio Napolitano had no such power, having been in the final months of his term of office.

rc/ccp (AFP, dpa, Reuters)