Brown told delegates to the Major Economies Forum talks in London that failure countries need to compromise with one another to reach a deal at December's summit to avoid "the catastrophe of unchecked climate change."
He said nations must make substantial new cuts in gas emissions by 2020 to meet targets proposed in The Stern Report, which predicted economic breakdown if climate change is not controlled.
The Copenhagen summit caps two years of negotiations on a global climate change treaty to replace the U.N.'s 1997 Kyoto Protocol on carbon dioxide emissions.
Brown, who plans to attend the summit personally, called on fellow leaders to join him.
"There are now fewer than 50 days to set the course of the next few decades," Brown said. "We cannot afford to fail. If we fail now, we will pay a heavy price ... If we falter, the Earth will itself be at risk."
Few other world leaders have said they would go to the summit, though Danish Prime Minister Lars Loekke Rasmussen and U.N. Secretary-General Ban Ki-moon will likely attend.
Wealthy nations are seeking broad emissions cuts from all countries in the new pact.
Developing countries say industrialized nations should carry most of the burden, saying tough limits on their emissions would likely hamper their economic growth.
The British leader appealed to both the industrialized and developing world, saying they each could take advantage of business opportunities in creating new energy sources and improving energy efficiency.
The two-day talks in London, ending Monday, were attended by representatives from Australia, Brazil, Canada, China, Denmark, the EU, France, Germany, India, Indonesia, Italy, Japan, Korea, Mexico, Russia, South Africa, Sweden, Britain and the United States.
Pressure has been mounting for the U.S. to finalize its position before the December summit. The Obama administration says it is tied to action by Congress, where climate bills are slowing moving forward.
Other nations including India, China, Brazil and Mexico have agreed to draw up national programs to slow the growth of greenhouse gas emissions, but have resisted making those limits binding and subject to international monitoring in a treaty.
Worries over the U.S. and China have led to mounting pessimism that a deal can be struck in Copenhagen without major policy changes.
"The prospects that states will actually agree to anything in Copenhagen are starting to look worse and worse," Rajendra Pachauri, head of the U.N. scientific panel studying climate change, wrote in a Friday post on the Newsweek Web site.
However, Sweden's environment minister, Andreas Carlgren - who represented the EU at the talks in London - told The Associated Press he has become more optimistic about the chances for an agreement in Copenhagen. He said progress had been made in talks over how to manage a system for funding climate measures.
"We are more in agreement about how the financial system should be managed, and there is also shared support for the idea that developing countries should be included and have their say in the management of the system," he said.
Most estimate that hundreds of billions of dollars would be needed every year.
The London meeting - the second meeting of the Major Economies Forum - was seeking agreement on funding from the developed world for poorer countries, to help them adapt to changes in the earth's climate that threaten to flood coastal regions, make farming unpredictable and spread diseases.
President Barack Obama launched the informal forum this year to privately discuss key international problems.
For more info:
Major Economies Forum on Climate Change
United Nations Climate Change Conference (COP15) - Copenhagen, Dec. 7-18, 2009
Report: The Stern Review on the Economics of Climate Change (hm-treasury.gov.uk)
Intergovernmental Panel on Climate Change
"Cutting the Cost: The Economic Benefits of Collaborative Climate Action" (The Climate Group)
Summary of American Clean Energy and Security Act of 2009
By Associated Press Writer David Stringer; AP Writers Malin Rising in Stockholm and Gregory Katz in London contributed to this report