By Richard Black
Environment correspondent, BBC News website, Barcelona
Speculation has been rife that nations' deals would not be binding
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, Mr 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.
"We would have preferred a full legal treaty, it has to be said," said Mr Miliband.
"I think the important thing about the agreement we now seek in December is that while it may be a political agreement it must lead, on a very clear timetable, to a legally binding treaty.
"Also, I'll be completely clear about this: I think an agreement without numbers is not a great agreement. In fact it's a wholly inadequate agreement."
It remains unclear whether the US could put numbers forward in Copenhagen - on reducing emissions, or on financing for poorer countries - in the absence of domestic legislation.
Several EU delegates to the preparatory talks here in Barcelona - at the final round before the Copenhagen summit - say that the complexity of the treaty means that drawing up all the legal ingredients is just too big a task to be finalised this year.
But South Africa's Alf Wills, who co-ordinates the G77/China bloc of developing countries on extending the Kyoto Protocol, suggested the real hurdle was political rather than logistical.
"We've got text - what we don't have is agreement on which parts of the text are the way to go," he told BBC News.
He also rejected suggestions by some developed nations that major developing countries had been remiss in putting forward proposals for reducing the rate at which their carbon emissions rise.
"China has published a five-year plan, India has published proposals, as has Brazil - and a few weeks ago Indonesia said it would cut the rate of growth of emissions by 40%, doing 26% of that by itself (without outside aid)," he said.
"So the statement (that developing countries have not put proposals forward) is not a statement of fact."
Activists strung a banner from Barcelona's Sagrada Familia church
Other European delegates agreed that a legally binding deal was very unlikely this year, but said that did not mean that nothing would happen.
"A lot of people still think that we can do something that will lead to real implementation in the fight against climate change - we will spend money, we will enact legislation, we will continue in this," said Artur Runge-Metzger, chief negotiator for the European Commission.
As to when all the loose ends should be tied up, he suggest three to six months was a reasonable period.
However, delegates close to developments in the US Congress said US legislation might not be finalised within six months.
Environment groups suggested western countries had not invested enough political energy in the process.
"Copenhagen is one of the most important meetings in human history, but the politicians seem determined to blow it," said Joss Garman of Greenpeace.
"So much can blamed on the Big Carbon special interests driving Washington. If Europe doesn't stand up to America to save this deal, there could be grave implications for millions across the world."