The UK government says it is highly unlikely that a new legally binding climate treaty can be agreed this year - and a full treaty may be a year away.
Two years ago, the world's governments vowed to finalise a new treaty at next month's climate summit in Copenhagen.
Climate Secretary Ed Miliband has until now said it could be done - but now he says only a political deal is likely, echoing some other senior figures.
Developing countries reacted with frustration and disappointment.
"When we left (UN talks in) Bali two years ago, we all expected that would be agreeing on a legally binding outcome to respond to the urgency... that we were on the verge of catastrophic climate change, so we're very disappointed," said Selwin Hart from Barbados, speaking for the group of small island developing states.
"If we don't take urgent and ambitious action, the reality is that some small island developing states will not be around within a couple of decades - certainly not by the end of the century."
This is thought to be the first time that UK ministers have acknowledged the slim chances of achieving anything legally binding.
In the middle of October, Miliband said a new treaty looked "more do-able" following a meeting of the Major Economies Forum in London.
His comments now echo warnings from UN Secretary-General Ban Ki-moon, Denmark's Prime Minister Lars Loekke Rasmussen and US chief climate negotiator Todd Stern that only a "politically binding" agreement can now be achieved.
Officials then warned it could take up to a full year to finalise the treaty.

