"It's a very strong political signal that despite the financial crisis no state wanted to change the deadline or the objectives" of cutting greenhouse gases by a fifth, he said.

Donald Tusk, the Polish prime minister, is currently hosting a UN climate conference where delegates from more than 180 countries are seeking to find agreement on a new treaty to replace the Kyoto Protocol.

Poland relies on high-polluting coal for more than 90 per cent of its electricity and has threatened to veto an EU plan to cut greenhouse gas emissions by 20 per cent below 1990 levels by 2020 unless it wins fossil fuel concessions.

Following talks with Sarkozy, Tusk said: "There is still a lot of work ahead of us [before the December summit]."

Poland argues it needs until 2020 to curb carbon emissions, for example by using more efficient boilers and carbon-scrubbing equipment and possibly building its first nuclear plant.

Mandatory purchases

Tusk said Sarkozy and the EU commission had agreed to extend a period limiting mandatory purchases of greenhouse gas emissions permits for east European coal plants, in an offer which would need the backing of all EU leaders.

Hinting at a willingness to compromise at the summit, Tusk said: "At the very end, maybe at the very last minute, we may decide this is a solution we may accept."

East European nations are not the only ones with objections.

Silvio Berlusconi, the Italian prime minister, has called the EU plan unrealistic and Rome has raised the possibility that EU leaders may approve only parts of the package at the summit.

Poorer east European states say tough caps on carbon emissions will harm their economies at a time of global financial crisis, preventing them from catching up with richer western Europe.

"The effects of the economic crisis have been very strong on our countries which are weaker than the western states," Calin Tariceanu, the Romanian prime minister, said.

EU diplomats are keen to wrap up climate talks this month before France hands the EU's presidency to the Czech Republic.