Prodi, a former European Commission president, has made much of his five years in Brussels where he stood on a world stage and earned a reputation for seriousness -- a campaign motto he coined to contrast with Berlusconi's flamboyance.

He saw EU expansion and the introduction of the euro to fruition, but had only limited success in his attempts to stamp out waste and fraud. He left many cold with his lack of communication skills and presence.

While waiting for Berlusconi to step down, Prodi has been working with his partners to try to put together a cabinet.

Prodi managed to get his candidate elected speaker of the Senate at the fourth attempt on Saturday, but the drawn-out vote revealed coalition cracks that bode ill for future stability.

Prodi made light of the problems ahead after his man finally clinched victory with 165 votes in the 322-seat Senate, against 156 for the centre right's candidate, 87-year-old elder statesman Giulio Andreotti.

"Today we have a positive situation and I believe the legislature can begin well," Prodi told reporters.

Earlier on Saturday, veteran communist leader Fausto Bertinotti, was elected president of the Chamber of Deputies.

Prodi's spokesman Silvio Sircana told Reuters the election of speakers for both parliamentary houses had forced Berlusconi to quit. "It's just normal procedure. He was obliged to resign at the first reasonable opportunity," said Sircana.

Ciampi would normally have the task of asking Prodi to form a government, but his term of office expires in mid-May and he has signalled the job should fall to his successor. Prodi would like to be nominated before then.