Greek conservative leader Antonis Samaras will send a letter of commitment to the terms of an EU/IMF bailout deal within the day, a party source said on Wednesday, with the country's bankruptcy rescue hanging in the balance.

Euro zone finance ministers had cancelled face-to-face talks on the 130-billion-euro ($170 billion) deal on Wednesday, saying they had yet to receive written pledges from Greek political party leaders to stick to punishing spending cuts, or clarification of all the savings.

The letter will be dispatched within the day, a New Democracy party source told Reuters on condition of anonymity. Doubt has focused on Samaras, Greece's likely next prime minister and a strong critic of the austerity measures.

Ministers of the Eurogroup downgraded the Brussels meeting -- originally planned for 1700 GMT -- to a conference call.

Greece needs the funds by next month to avoid a messy bankruptcy.

The Eurogroup is next due to meet on Monday, but Greece has said it must initiate a debt swap deal with private sector bondholders by Friday if it is to meet a March 20 for 14.5 billion euros in debt repayments.

The Eurogroup had asked Greece to clarify how it would fill a 325-million-euro gap in budget cuts promised for 2012 and to persuade all party leaders to sign a commitment to implement austerity measures after an election expected in April.

It's true we are asking the Greeks for some extremely painful sacrifices and I understand their anger, but Greece has made many errors in its past, French Foreign Minister Alain Juppe told France Info radio on Wednesday.

It (the bailout) must be concluded because if Greece went bankrupt and left the euro zone, the chaos would be even worse for the Greek people and very bad news for the euro zone.

The cabinet of Prime Minister Lucas Papademos discussed the gap in the 3.3-billion-euro austerity program late into Tuesday evening. But there was no official clarification of where the savings would come from.

ACCELERATING CONTRACTION

Samaras has criticized the punishing new cuts in wages, pensions and jobs adopted by parliament early on Monday as rioters wrecked buildings across central Athens.

He voted for the bill, and expelled a quarter of his deputies who did not. But he says the cuts could plunge the country, already in its fifth year of recession, into an even bigger slump.

When parliament debated the austerity package on Sunday Samaras indicated that he would try to renegotiate the terms of the bailout, increasing doubt in the minds of European leaders.

The timeline to 20 March is getting tighter, but Europe is still more likely than not to rescue Greece from disorderly default, Christian Schulz, senior economist at Berenberg bank, said in a note.

Some EU leaders have suggested Athens should leave the euro zone currency union.

The EU and IMF want Greece to account for every cent of budget cuts before they approve the rescue, which includes a bond swap, cutting the real value of private sector investors' bond holdings by some 70 percent.

But Greece's downward economic spiral has accelerated. Data on Tuesday showed that the economy shrank by seven percent in the fourth quarter of last year, even more than the five percent contraction of the third quarter.

Greece is well on its way to suffering one of the biggest slumps of modern history. Gross domestic product has contracted 16 percent from its peak and the austerity will make that worse.

Papademos has said that failure to back the bailout would consign Greece to economic catastrophe.

But with many Greeks suffering huge cuts in their living standards and young people burning and wrecking almost 100 Athens buildings in one night on Sunday, some people believe the catastrophe is already under way.

On the current path - which is not sustainable in my view - we may very well see Greek GDP go down 25-30 percent, which would be historically unprecedented. It's a disastrous crisis for them, said Uri Dadush, at the Carnegie Endowment think tank in Washington.

That would put Greece in the same league as the United States, where the economy shrank 29 percent during the Great Depression.

($1 = 0.7616 euros)

(Additional reporting by Renee Maltezou in Athens, Alan Wheatley and Scott Barber in London, Aileen Wang in Beijing; Writing by matt Robinson)

International Monetary Fund