British Prime Minister Gordon Brown was to warn representatives of the world's biggest carbon polluters Monday of a climate catastrophe if they do not strike a deal at the Copenhagen summit. Brown was due to address the Major Economies Forum meeting in London with a message that they must find a way to 'break the impasse' on getting a far- reaching agreement at the United Nations climate conference in December. The 17 powers that make up the MEF, along with developing nations and UN representatives, are trying to iron out some of their differences before the crunch summit in Denmark. 'In every era there are only one or two moments when nations come together and reach agreements that make history -- because they change the course of history,' Brown was to say, according to extracts from his speech released by his Downing Street office. '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.' The MEF was launched by US President Barack Obama earlier this year on the back of an initiative by his predecessor, George W. Bush, to speed up the search for common ground among the most polluting world economies. The December 7-18 UN climate summit in the Danish capital will see nations attempt to hammer out a new global climate treaty to replace the Kyoto Protocol, which expires in 2012. Brown was to say he thought a deal at Copenhagen was possible, but negotiators were not reaching an agreement fast enough. Instead, 'leaders must engage directly to break the impasse. We cannot compromise with the Earth,' he was to say. 'We can not compromise with the catastrophe of unchecked climate change; so we must compromise with one another. 'We cannot afford to fail. If we act now; if we act together; if we act with vision and resolve, success at Copenhagen is still within our reach. But if we falter, the Earth itself will be at risk. 'This is the moment. Now is the time. For the planet there is no Plan B.' 