The Prime Minister said there would be a global "catastrophe" if action to tackle climate change was not agreed at United Nations talks in Copenhagen in December. He also insisted "there is no plan B".
The conference will bring together environment ministers from 192 nations to try to reach an agreement on a deal to curb greenhouse gas emissions.
Mr Brown said the economic cost of failing to address global warming would be greater than the impact of the two world wars and the Great Depression. And he warned Britain could be hit by heatwaves, droughts and flooding if climate change was allowed to rise unchecked.
It is widely accepted that a global temperature rise of more than 2C will cause environmental chaos.
The Copenhagen summit, which begins in 49 days, on 7 December, is seen as the last chance for world leaders to agree how to tackle the crisis.
Read energy and environment consultant Richard Courtney's opinion here
Speaking yesterday to representatives of 17 countries, including the United States and China, Mr Brown said: "In every era, there are only one or two moments when nations come together and reach agreements that make history – because they change the course of history. Copenhagen must be such a time. There are now fewer than 50 days to set the course of the next 50 years and more."
The people least responsible for climate change – those in the world's poorest countries – were being hit hardest, he said, with the effects of drought, floods, loss of farming and the spread of disease already killing 300,000 people a year.
However Britain, too, would suffer, with droughts, flooding and heatwaves similar to the 2003 event that led to the deaths of 35,000 people across Europe, the Prime Minister warned.
Green groups backed Mr Brown's warning of a catastrophic future for the UK and the world if a deal on cutting climate emissions was not secured in Copenhagen, but opposition MPs claimed there was a gap between his rhetoric and action.
Mr Brown told the Major Economies Forum, gathered in London for the second day of talks, that he believed a deal in Copenhagen was possible but that countries were not making progress quickly enough.
"We cannot compromise with the Earth," he said. "We cannot compromise with the catastrophe of unchecked climate change; so we must compromise with one another."
He called on world leaders to work together to achieve a deal that set out binding targets for rich countries to cut their emissions, and that agreed a package of funding to help the poorest countries cope with the impact of climate change. "We can't afford to fail," he said. "If we fail, we pay a heavy price. For the planet, there is no plan B."
Mr Brown went on: "If we do not reach a deal at this time, let us be in no doubt: once the damage from unchecked emissions growth is done, no retrospective global agreement in some future period can undo that choice.
"By then, it will be irretrievably too late. So we should never allow ourselves to lose sight of the catastrophe we face if present warming trends continue."
Mr Brown is one of the few leaders of major economies who has pledged to attend the Copenhagen talks in person to try to secure a deal. Yesterday, he urged other leaders to follow suit.
The Copenhagen summit aims to secure a new deal to cut global greenhouse gas emissions that are driving climate change, replacing the Kyoto Protocol.
With only one more week of UN talks in Barcelona before the meeting, politicians risk an impasse over the role of developed versus developing nations, and the level of financial support required from wealthy countries.
Mr Brown's words coincided with the launch of an inquiry by the Royal Society of Edinburgh into public acceptance of the need to tackle climate change.
Dr Sam Gardner, climate change policy officer at WWF Scotland, welcomed the Prime Minister's comments. "Certainly, his comments reflect the level of leadership we need to see," he said. "He very much appears to recognise the level of urgency.
"His commitment to attend Copenhagen is important and we hope to see other world leaders commit to being there, particularly President Obama who will be in Scandinavia around the same time to collect his Nobel Peace Prize. We have a very small window of time, in terms of agreeing a deal but also in terms of cutting emissions."
Mr Gardner believes the European Union must commit to cutting emissions by 40 per cent by 2020, and agree a finance package of at least 100 billion (91bn) by 2020.
Friends of the Earth executive director Andy Atkins said rich nations, such as Britain, held the key to success in the talks and urged them, as the cause of much of the problem, to take the lead in finding solutions.
"So far, developed countries have not delivered: they have not promised to cut their emissions by at least 40 per cent by 2020 without carbon offsetting, nor have they pledged sufficient funds to enable poor nations to develop cleanly and adapt to the impacts of climate change," he said.
He said the next few weeks were "crucial in determining the long-term future of the planet", adding: "The world must pull back from the brink and take urgent action to slash emissions."
Julian Oram, head of policy at the World Development Movement, warned of a gap between Mr Brown's rhetoric and UK policy, accusing the government of "complete inertia" on concrete actions to cut emissions.
And the Liberal Democrats' Simon Hughes said: "Expansion of the UK's largest airport and continued plans for dirty coal power stations are not the policies of a government with the authority to lecture the world on averting the climate crisis."
Expert: We must 'sell' message
A MAJOR inquiry into Scotland's role in tackling climate change was launched yesterday by the Royal Society of Edinburgh.
The inquiry will look at the gap between the policies necessary to deal with climate change and what the Scottish public will currently accept.
The committee will be headed by climate scientist Professor David Sugden, of Edinburgh University, a world leader in research on ice sheets.
As part of the 90,000 inquiry, which will report in early 2011, the committee will hold regional debates, a schools competition on a low-carbon future and public lectures.
Prof Sugden told The Scotsman: "We want to home in on the issue of how we get the public to buy into what we need to do to achieve a low-carbon economy.
"The evidence and the need to move towards a low-carbon economy is well established, but people, I think, have not quite bought into the changes that are needed."
He believed there were two barriers to change: the sheer scale of the problem and strong campaigns "adding doubt to the science".
The remit of the inquiry – Facing Up To Climate Change – will be to engage in dialogue, to identify barriers to change and to recommend policy measures in those areas.
He said that achieving the ambitious targets set by the Scottish and UK governments, such as generating 50 per cent of electricity from renewables by 2020, would involve huge changes.
However, he added that Scotland was in a strong position to make those changes.
"You can argue that if we can't make the changes, then what hope is there for poorer countries," he said.
"We must tackle the problem now. It is no use saying we will tackle the problem when it comes.
"I'm a grandfather with five grandchildren, and what are we going to leave them with?"