Brown warns of climate catastrophe

In a bleak message to urgent talks between 17 leading nations in London, he will say there is "no plan B" if agreement is not reached at December's UN-sponsored summit in Copenhagen.

With less than 50 days to go until the crunch meeting, serious fears remain that international wrangles over emission cuts targets will end in deadlock.

Mr Brown, who has pledged to attend the summit in person in a bid to end the impasse, will renew his call to fellow world leaders to speed up efforts to iron out differences.

"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," he will tell the Major Economies Forum.

"By then it will be irretrievably too late," he will say. "So we should never allow ourselves to lose sight of the catastrophe we face if present warming trends continue."

By 2080 an extra 1.8 billion people - a quarter of the world's current population - could lack sufficient water, he will warn, but insist it is not just poor countries that are affected.

"The extraordinary summer heatwave of 2003 in Europe resulted in over 35,000 extra deaths.

"On current trends, such an event could become quite routine in Britain in just a few decades' time. And within the lifetime of our children and grandchildren the intense temperatures of 2003 could become the average temperature experienced throughout much of Europe," he will say.

Agreement at Copenhagen "is possible", he will conclude.