British Prime Minister Gordon Brown warned on Monday the planet faced "catastrophe" if action to tackle climate change is not agreed at a key UN conference on global warming in December.
Brown told a meeting of 17 major developed and developing countries in London that the costs of failing to address global warming would be greater than the impact of two world wars.
He told the Major Economies Forum (MEF) that a deal at December's conference in Copenhagen was possible, amid recent warnings from the US 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.