The European Union has agreed a £6.5 billion three-year package to fast-track the response to global warming in the poorest countries, of which Britain will provide £1.5 billion, it was announced today.
Prime Minister Gordon Brown upped the British figure at the last minute from £1.2 billion to encourage bigger offers, particularly from Spain and Italy.
The move worked - delivering a final figure above the target for agreement at the summit, with Mr Brown putting in the largest national share.
France and Germany pitched in with about £1.2 billion each over three years.
Asked why the British contribution was the biggest, Mr Brown said: "I think we have done the right thing. This offer is one of the ways we can get a global agreement at Copenhagen."
He said that because of its Commonwealth background, Britain was historically committed to helping poor nations, particularly in Africa - and he predicted that Germany and France would also increase their contributions to the so-called "fast start" fund to help the developing nations meet their share of the financial burden of a global deal.
The PM also confirmed that EU leaders meeting in Brussels have agreed they will offer a 30% cut in greenhouse gas emissions by 2020, if other countries are willing to commit themselves to ambitious reductions in Copenhagen.
This offer falls short of Mr Brown's call earlier this week for the EU to commit itself to 30% cuts regardless of what other developed countries do.
Mr Brown said today's unanimous decisions amounted to "a very significant move forward in the search for a Copenhagen agreement".
Europe wants a 10 billion US dollars-a-year (£6.14 billion) fund to be set up for the period 2010/12 to allow the developing world to take immediate action on mitigating emissions and adapting to the impact of global warming in the period before whatever agreement is reached in Copenhagen takes effect in 2013.
Today's promise of 2.4 billion euros (£2.17bn) a year is more generous than the 2 billion euros expected before the Brussels meeting of the European Council. It amounts to around 3.6 billion dollars and meets more than one-third of the total fund.
Speaking at a press conference in Brussels, Mr Brown said: "Britain's contribution is one that we are proud we are making, and that is 800 million dollars or £500 million a year.
"That makes it possible for the poorest countries to come to the table knowing that they can mitigate carbon emissions and adapt to climate change."
Cash from the fast-track fund will go towards changes to help poor countries deal with the impact of global warming, such as sea walls, hurricane defences and low-water agricultural techniques, as well as measures to reduce carbon in the atmosphere, such as protecting rainforests and cutting emissions.
"This means that Europe is contributing its best towards the Copenhagen settlement over these next few days," said Mr Brown.
The PM said EU leaders had agreed to do "everything in our power" to achieve an ambitious outcome to the UN-sponsored Copenhagen summit, which is trying to put a framework in place for the period after the Kyoto climate change agreement expires in 2012.
The EU was "prepared to go to 30% as a cut in carbon emissions as the EU, as long as we have ambition from other countries as well", said Mr Brown.
And he said the EU would back moves to put a 100 billion dollar fund in place by 2020 to help poor countries meet the challenges of climate change.
Mr Brown said: "I will do my best, working with other countries, to contribute to the success of the Copenhagen process.
"I believe there is goodwill now, that there is a determination that things move forward. Obviously we are hoping that other countries respond to the generous offers that Europe has made but we look forward to a successful outcome."
The cash pledge meant developing nations could now "see that their needs are being taken seriously", he said, adding that he hoped to go to Copenhagen early next week to meet with African and other developing countries' representatives.
He said he would also co-chair, with French President Nicolas Sarkozy, a meeting with African nations to discuss a deal on deforestation.
The development agency ActionAid questioned today's EU cash offer, claiming that "almost all of the money is likely to be simply a relabelling of existing aid commitments".
ActionAid's EU expert, Anne-Catherine Claude, said: "The key test of whether money is truly new is whether it is additional to the 0.7% aid target that rich countries signed up to 39 years ago.
"None of the money announced today appears to meet that test."
Mr Brown rebutted that at his post-summit press conference, insisting the UK contribution was not a recycling of existing funds.
He said: "We had always made provision for a contribution to the Copenhagen climate change deal (in the Overseas Development Aid budget): it is not a surprise to us that we have got to come up with early-start finance."
And he pointed out that, from 2013, all UK financial contributions to the longer term costs of a global deal to cut CO2 emissions would be additional to the existing 0.7% minimum share of GDP that is committed to development aid.
He said the issues for the EU summit had been:
* "Are we going to get a climate change deal at Copenhagen? - Yes we are.
* "Has Europe stepped up to the mark? - Yes it has.
* "Has the UK taken a leadership role? - "That is what we are doing."
But Friends of the Earth said the chances of reaching a "just and fair" international climate change deal were "extremely slim" because the EU had failed to significantly strengthen its position for the Copenhagen meeting.
Sonja Meister, FoE climate campaign coordinator, said: "This EU summit has not given any reason to be more hopeful. The EU's decision not to commit for certain to more than 20% emissions cuts by 2020 is insufficient and unjust.
"EU leaders come to Copenhagen next week and now there is even more pressure than ever for them to commit to 40% domestic emissions cuts in line with what science and climate justice demand."
She added: "Providing fast track money cannot excuse the EU from its real long-term commitments.
"That the EU is largely using money which otherwise would have been used to build schools or deliver health care in the developing world is shameful and will not be welcomed by developing countries in Copenhagen."