Gordon Brown has warned it may not be possible to get a deal in Copenhagen, where more than 190 countries are attempting to thrash out a new agreement on tackling climate change. But what are the sticking points between countries?
* Finance: Poor countries have done least to contribute to the greenhouse gas emissions that are causing global warming, but the most vulnerable are the ones who are going to be hit hardest and first by its impacts.
For developing countries to tackle climate change, they will need help from rich nations to enable them to develop without increasing their pollution, through "mitigation" measures including renewable power, as well as "adaptation" cash to help them become resilient to impacts such as floods, droughts, storms and loss of crops.
Prime Minister Gordon Brown has proposed a "fast start" fund of 10 billion US dollars a year over the next three years, but he has also estimated that 100 billion US dollars in public and private funding is needed a year by 2020.
Developing countries want reliable, predictable sources of funding over the long term, which is not raised by raiding existing aid budgets - and they are concerned the fast-start money is evidence of rich countries trying to wriggle out of long-term pledges.
While a number of countries and blocs, including the EU, put forward proposals for the kind of financing that might be delivered and mechanisms to deliver it ahead of the UN talks, draft versions of the text for possible agreements have currently got no numbers in them.
And UK officials have admitted that there may not be firm numbers in the agreement.
* Emissions reductions: Negotiators and leaders at the UN talks have accepted that we have to cut emissions if we are to avoid dangerous climate change. Globally, emissions must peak in the next decade and decline by at least 50% by 2050 to have a chance of keeping temperature rises under 2C.
Some countries, including low-lying islands, have been demanding that the world aims to keep temperature rises to 1.5C in the "shared vision" set out at the beginning of any new agreement - but 2C is a more likely target.
So far, a number of countries - including the US, EU and China - have put forward offers for the kind of curbs on emissions they are prepared to make.
But added together, the ambition is not enough to deliver the level of cuts the scientists say is needed, and many of the pledges are conditional on other countries' actions and an ambitious deal being achieved at Copenhagen.
For example, the EU has said it will increase its offer to 30% cuts by 2020, instead of unilateral reductions of 20% on 1990 levels, but wants to see more ambition from the US and China, who have not budged on their offers.
* Monitoring: The other problem with emissions cuts is that they need to be checked to ensure the promised reductions are being made. This process, known as MRV - monitoring, reporting and verification - is controversial because major developing economies such as China and India do not want detailed outside scrutiny on their economies.
The US is among those countries which are demanding that promises of emissions curbs are backed up with evidence - especially if developing nations are to receive funding for their actions, although China has said it does not want money for the steps it is taking.
* The deal: While it was originally hoped that a signed, sealed and delivered legally-binding international treaty could be agreed by the end of the talks in Copenhagen - after two years of negotiations - that is no longer a possibility.
Leaders are now working towards a "political agreement" with the legal treaty to come later, but just how much can be agreed and how much will have to be thrashed out over the coming months has yet to be seen.
UK politicians are keen for the political agreement to include a timetable for the drawing-up and signing of the legal document within months, and former US vice-president Al Gore has called for another conference to secure a treaty in July in Mexico.
There is also the complicating factor of the existing climate agreement, the Kyoto Protocol, which some countries want to keep as it is currently the only deal which commits rich nations to legally-binding emissions, and do not want it ditched in favour of a non-legally-binding political agreement.
But the US has refused to ratify Kyoto, and it does not include measures by developing countries such as China - now the world's largest polluter - while financial support for poor countries and action on deforestation are also missing.
While the UK was keen to see one deal agreed, with the provisions of Kyoto included, negotiations are continuing on "twin tracks" with talks on the next phase of Kyoto, which is supposed to begin in 2013 alongside meetings on a wider agreement.