Gordon Brown warned today of a catastrophe for the planet if action to tackle climate change is not agreed at forthcoming UN talks and said that global warming could be more costly than two world wars and the Great Depression.

The Prime Minister delivered the analysis in a speech to representatives from 17 countries at the Major Economies Forum (MEF), convened as part of efforts to secure a deal at the UN summit in Copenhagen in December.

Mr Brown examined the economic, human and ecological impact of a failure to cut the emissions blamed for driving up temperatures.

He also told the forum, which is gathered in London for a second day of talks, that he believes a deal in Copenhagen is possible, but with fewer than 50 days to go governments were not making progress quickly enough.

Mr Brown called on world leaders to work together directly to achieve a deal that 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."

He added: "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 has said that he plans to attend the Copenhagen summit, which will cap two years of negotiations on a global climate change treaty to succeed the Kyoto Protocol. He has called on fellow leaders to join him, although so far few have said that they will.

In today's speech 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 pointed out that President Nasheed of the Maldives held a Cabinet meeting under water on Saturday to highlight the catastrophe facing his country and that the South Pacific nation of Kiribati was requesting international aid to evacuate the islands before they are permanently flooded.

But Britain too would be hit by the impacts of climate change in the coming decades, including heat waves similar to the 2003 event that led to the deaths of 35,000 people across Europe, droughts and flooding.

Mr Brown said that some effects of climate change were already "inescapable" but action to cut greenhouse gas emissions could slow the rate of change to a pace that would enable people to adapt. He acknowledged the "formidable political constraints and challenges" in securing a deal, but said that 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 mid-term — 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 that are driving climate change. Developing countries want richer nations, which they point out are responsible for the vast majority of harmful emissions to date, 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.

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