By Philip Pullella

ROME (Reuters) - Italian Prime Minister Silvio Berlusconi's government lost a key parliamentary vote on Tuesday and some of his center-right leaders said a confidence vote would be necessary to ensure the coalition still had a workable majority.

The opposition demanded Berlusconi resign after the government lost by a single vote on a measure to formally approve last year's balance sheet for state spending.

"There is no reason to see this as a collapse. But I think that the government needs to be willing to be open to debate and see if there is a political consensus," said Fabrizio Cicchitto, head of center-right deputies in the lower house.

Defense Minister Ignazio La Russa said the government should call a confidence vote to prove that the coalition was still intact and that the loss of the vote was just a mishap because of some absences in the chamber.

But Gianfranco Fini, the lower house speaker who broke with Berlusconi last year, said the loss of the vote was "unprecedented" in its seriousness.

The government was defeated because several key center-right leaders, including Economy Minister Giulio Tremonti and Northern League leader Umberto Bossi, were not in the chamber for the vote.

Still, the defeat was stinging because it comes at a time when Berlusconi is facing internal challenges from a number of center-right ministers who are unhappy with the way he is running the coalition and the damage his personal and judicial woes have done to Italy's reputation.

The opposition called on Berlusconi to face the fact that he no longer had a workable majority and step down.

"This government has no program left, it has no coalition, it has no objectives except to guarantee itself power," said Massimo Donadi, head of parliamentarians in the opposition Italy of Values party.

The 75-year-old prime minister has come under mounting attack as the financial crisis and growing divisions in his center-right coalition fuel speculation that his government will collapse before the end of its term in 2013.

Ratings agency Fitch last week cut Italy's credit rating by one notch with a negative outlook, following a downgrade by Moody's, underlining market concern over the stability of its public finances and its chronically weak growth.

A 60-billion-euro austerity package to balance the budget by 2013 was passed last month only after weeks of hesitation and delay, while the timetable for a decree to pass economic reforms and approve the sale of state assets has slipped to October 20.

So far, Berlusconi's majority in parliament has held up in repeated confidence votes but there has been mounting press speculation of a possible revolt within his PDL party.

(Additional reporting by Deepa Babington)