The Prime Minister spoke as Governments meeting in Denmark failed to resolve their disputes over how much to cut their emissions, how to prove that those cuts are actually made, and who should pay for the move to a low-carbon economy.
As British officials admitted that the talks remain “ very difficult”, Ed Miliband, the climate change secretary, signalled Britain is prepared to back a move to hold another international climate summit in Mexico City next summer, several months ahead of schedule.
The prospect of bringing forward the Mexico meeting was first made by Al Gore, the former US vice president and environmentalist.
Mr Miliband said it was possible that leaders will leave Copenhagen without resolving some of the biggest issues on the table. He said: “Can I guarantee that everything is going to be settled by the end of this week? I can’t guarantee that.”
Achim Steiner, the head of the UN Environment Programme, said that without a real deal, it could be better to defer big decisions until the next summit.
Mr Steiner said: “A meaningless deal in Copenhagen cannot be in anyone’s interests because it locks us into another decade of
inadequate action and co-operation so if time runs out there is always the option of stopping the clock and reconvening to get it right. “But the risk is that that the momentum that is so characteristic of these 10 days in Copenhagen might be lost and then the world will struggle to take this further in the next six to 12 months.”
Mr Brown insisted that he could “see a way through” the disputes, but accepted the possibility of failure.
He said: “I don’t agree with the proposition that people have given up on this meeting at all. Of course, there is a possibility of a deadlock, of course there is a possibility that people will find it difficult to come to an agreement.”
Among the major issues still undecided is how much the US will reduce its carbon emissions.
At the moment the US has pledged to cut emissions by 17 per cent on 2005 levels. This equates to around 4 per cent on 1990 levels and there is increasing pressure at the Copenhagen climate summit for the US to raise its target.
Mr Brown said he wanted the US to offer bigger cuts. “The American offer is a very significant reduction in a very short period of time, but everyone is being asked to set a higher level of ambition”, he said.
However, US officials in Copenhagen have ruled out any increase, British officials privately accepted that any increase in the American offer is very unlikely.
The Copenhagen talks are also supposed to agree to a dollars 100 billion fund to finance carbon-reducing technologies in developing countries.
Yet here again, officials said an agreement on contributions remains elusive. One senior British source said that any text agreed on Friday might only set approximate ranges for rich countries’ contributions to the fund and not give precise numbers.
There is also a continued stand-off between China and the US, the world’s two biggest carbon emitters.
Jose Manuel Barroso, president of the European Commission, singled out the two for blame over the deadlock in negotiations.
All countries “need our American partners and Chinese partners to move more" if the deal is to be successful, he said at a press conference.
China has pledged to cut emissions by 40 per cent on “business as usual” – although this actually allows for an increase due to the rate of the superpower’s growth.
Significantly, Beijing has also opposed independent international verification and reporting on its carbon emissions cuts.
At the summit, US Senator John Kerry insisted America will not cut greenhouse gas emissions until emerging countries like China also agree to monitoring.
Mr Kerry, who has been behind efforts to pass climate legislation in the Senate, said any new climate deal will only pass through the US Senate if American workers know there are similar limits on industry in the East.
He said: "To pass a bill, we must be able to assure a senator from Ohio that steel workers in his state won't lose their jobs to India and China because those countries are not participating in a way that is measurable, reportable and verifiable," he said.
"Every American - indeed, I think all citizens - need to know that no country will claim an unfair advantage."