Charles Dallara, managing director of the Institute of International Finance (IIF), said if EU leaders imposed steeper losses on bondholders than the 21 percent write-down they had accepted in July, it could prompt investors to sell other countries' sovereign debt, destabilizing the single currency.

"We do not see that a compelling case has been made to reopen the deal," Dallara said in Saturday's Financial Times. "A deal is a deal."

A spokesman for the IIF said Dallara was in Europe to discuss the agreement and would continue talks next week.

European Economic and Monetary Affairs Commissioner Olli Rehn played down Dallara's comments after a G20 finance chiefs' meeting in Paris, saying the Europe was not going back on the "voluntary" bond swap deal but merely updating it.

"We will certainly work on the basis of the agreement of July 21 and we will likely do some technical revision due to change in market circumstances. This is now work in progress," Rehn told reporters.

"So we are not reopening the deal, we're rather revisiting the deal."

Getting holders of Greek bonds to agree to take a loss was a crucial part of a 109 billion euro second bailout for Greece agreed in July. But the deal has not yet been finalized and there has been criticism that what is known as the "haircut" taken by creditors is not enough.

An IIF official told Reuters this week that based on current market prices the loss would equate to 39 percent, following a rise in Greece's risk profile.

Some countries want the loss to be more than 50 percent, however, and want the deal re-opened.

"Circumstances have changed for everyone, but the Greek program is back on track with the IMF," Dallara told the FT.

German finance minister Wolfgang Schaeuble said ministers would find a solution for Greece based on a debt sustainability analysis by the so-called troika of EU and International Monetary Fund inspectors, expected next Wednesday.

French Finance Minister Francois Baroin, asked about Dallara's comment, would only say: "We will find an answer. You know the French position which is quite clear: we will refuse any solution that leads to a credit event."

A mandatory debt restructuring would be classified as a credit event triggering credit default swap insurance payments and sending wider tremors around the financial system.