Gordon Brown warned today of a "catastrophe" for the planet if action to tackle climate change is not agreed at forthcoming UN talks on global warming.
Speaking to representatives of 17 countries at the Major Economies Forum, convened as part of efforts to secure a deal at the UN Summit in Copenhagen in December, the Prime Minister warned of the economic, human and ecological impact of a failure to cut the emissions driving up temperatures.
The costs of failing to address global warming would be greater than the impact of the two world wars and the Great Depression, he said.
He told the forum, gathered in London for the second day of talks, that he believes a deal in Copenhagen is possible.
But with fewer than 50 days to go before the UN talks, he warned them that countries were not making progress quickly enough to reach agreement.
He called on world leaders to work together directly to achieve a deal which sets out binding targets for rich countries to cut their emissions, action by developing nations and finance to help the poorest countries cope with the impact of climate change.
"We can't afford to fail. If we fail, we pay a heavy price," he warned.
"For the planet, there is no plan B."
Mr Brown said: "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."
He warned that the people least responsible for climate change - those in the world's poorest countries - were being hit hardest and first, with the effects of drought, floods, loss of farming and fishing yields and the spread of disease already killing 300,000 people a year.
He said President Mohammed Nasheed of the Maldives held a cabinet meeting underwater on Saturday to highlight the catastrophe facing his country, and the South Pacific nation of Kiribati was requesting international aid to evacuate the islands before they literally disappear.
But Britain too would be hit by the impacts of climate change in the coming decades, including heat waves similar to the 2003 event which led to the deaths of 35,000 people across Europe, droughts and flooding.
He said some impacts of climate change were already "inescapable" but action to cut greenhouse gas emissions could slow the rate of change to a pace which would enable people to adapt.
Mr Brown, who has pledged to attend the Copenhagen talks in person to secure a deal and has urged other leaders to follow suit, acknowledged the "formidable political constraints and challenges" in securing a deal, but said momentum was building towards success at the negotiations in December.
But with negotiations not moving fast enough - and with only one more week of UN talks in Barcelona before the Copenhagen meeting - he urged leaders to step in to break the impasse.
Progress must be made on efforts to provide cash for poor countries to develop without their emissions spiralling up to the levels seen in the West and to adapt to the impacts of climate change.
And binding targets for the midterm - such as 2020 - are needed from developed countries, along with action from developing nations and co-operation on low-carbon technology such as solar power and methods of trapping and permanently storing carbon from power stations.
The Copenhagen summit aims to secure a new deal to cut the global greenhouse gas emissions which are driving climate change.
Developing countries want richer nations, which they point out are responsible for the vast majority of harmful emissions historically, to commit to tougher targets - and not just in the long term.
They also want pledges of more cash to help them become greener themselves and to adapt to meet the challenges posed to them by the changing climate.
At the same time, fast-emerging economies such as China and India, which are among the biggest polluters in the 21st century, are under pressure to set out concrete proposals to limit the damage caused by their own rapid development.
There will be no formal outcome from the MEF meeting, which concludes today.
But the meeting, attended by representatives of 17 major economies and several nations particularly at risk from the impacts of rising temperatures, aims to narrow the gaps between countries on a number of issues to help progress towards a new deal.