LONDON (AP) - British Prime Minister Gordon Brown will warn representatives of the world’s biggest economies Monday that efforts to agree on a new global pact to tackle climate change are a historic test of international cooperation.

Brown planned to address the second day of Major Economies Forum talks in London, and tell delegates that any failure to strike a new deal on reducing the gas emissions causing global warming would be catastrophic.

The British leader plans to personally attend a December meeting in Copenhagen - intended to cap two years of negotiations on a global climate change treaty - and has called on fellow leaders to join him.

"In every era there are only one or two moments when nations come together and reach agreements that make history," Brown planned to say, according to excerpts of his speech released in advance. "Copenhagen must be such a time. There are now fewer than 50 days to set the course of the next 50 years and more."

Wealthy nations are seeking broad emissions cuts from all countries in a successor pact to the Kyoto Protocol on carbon dioxide emissions. Developing countries say industrialized nations should carry most of the burden, and complain that tough limits on emissions are likely to hamper their economic growth.