Brown told representatives from 17 top polluters at the Major Economies Forum (MEF) that the costs of failing to address global warming would be greater than the impact of two world wars.

He said a deal at December's conference in Copenhagen was possible, amid recent warnings from the United States that the meeting could fail in its goal to draw up a successor to the Kyoto Protocol.

But with fewer than 50 days to go before the UN talks, Brown warned that countries were failing to make progress quickly enough to reach agreement.

"We can't afford to fail. If we fail, we pay a heavy price," he warned.

"For the planet, there is no plan B."

Brown called on world leaders to work together directly to achieve a deal which sets out binding targets for rich countries to cut their emissions, and finance to help the poorest countries cope with the impact of climate change.

The prime minister 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."

Brown said he shared the concerns of low-lying island states such as the Maldives, whose government held an underwater cabinet meeting Saturday to focus global attention on rising sea levels that threaten to submerge them.

And he warned that the people least responsible for climate change -- the inhabitants of the world's poorest countries -- were being hit hardest and first.

The MEF comprises Australia, Brazil, Canada, China, the European Union, France, Germany, India, Indonesia, Italy, Japan, South Korea, Mexico, Russia, South Africa, Britain, and United States.

Its members represent about 90 percent of global emissions.

The London talks are focusing on emissions cuts, the protection of forests and climate finance -- Brown has said 100 billion dollars (70 billion euros) a year is needed to help developing countries tackle climate change.

Brown, however, said that "the signs of momentum, of forward movement, are now unmistakeable".

India said last month it was ready to set itself non-binding targets for cutting carbon emissions, while China said it would curb the growth of its emissions by a "notable margin" by 2020, although it did not specify further.

The US special envoy for climate change, Todd Stern, told British television on Saturday that developing economies must boost their efforts, warning it was "certainly possible" that no deal would be agreed in Copenhagen.

"What we need to have happen is for China and India and Brazil and South Africa and others to be willing to take what they're doing, boost it up some, and then be willing to put it into an international agreement," he said.

Greenpeace said it was concerned that the US was "threatening the possibility of a strong international climate deal" to keep global temperature rise below two degrees Celsius.

The group's climate political advisor, Martin Kaiser, called on the European Union to "stand behind its stated goals of saving the climate and reject US attempts to water down the entire agreement to suit its domestic agenda".

Climate campaigners Friends of the Earth said it was up to the rich countries in the MEF to "face up to their legal and moral responsibility by agreeing to cut their emissions first and fastest".