Mr Brown will urge leaders to break their "impasse"
Mr Brown will say there will be "no plan B" if agreement is not reached at December's UN summit in Copenhagen.
Negotiators have 50 days to save the world from global warming, he will add.
The Copenhagen summit in December is intended produce a new global climate change deal to replace the ageing Kyoto treaty.
But BBC deputy political editor James Landale said that not everything was going to plan.
At the meeting in London, the prime minister will warn that preparatory talks within the United Nations have reached an impasse（僵局，死路）.
Negotiators, he will say, are not reaching agreement quickly enough.
"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 tell the meeting that by 2080 an extra 1.8 billion people - a quarter of the world's current population - could lack sufficient water.
Mr Brown will say: "If we do not reach a deal at this time, let us be in no doubt: once the damage from unchecked emissions7 growth is done, no retrospective（回顾的，怀旧的） global agreement, in some future period, can undo8 that choice.
"So we should never allow ourselves to lose sight of the catastrophe we face if present warming trends continue."
Agreement at Copenhagen "is possible", he will conclude.
In recent days there have been a number of warnings that progress is stalling（失速，停转）, with Rajendra Pachauri, head of the Intergovernmental Panel on Climate Change, telling Newsweek magazine that "the prospects10 that states will actually agree to anything in Copenhagen are starting to look worse and worse".
MEF is not part of the formal UN process and so firm commitments are unlikely to come from the meeting.
It is seen instead as a forum where countries can explore options and positions in a less pressured environment.
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