But the Observer newspaper published Sunday excerpts from his 700-page report, which adds that unchecked global warming could make 200 million people refugees from drought or flood.

Publication of the report is likely to fuel debate in Britain over whether the government should introduce a tougher regime of "green taxes" to cut carbon emissions.

According to the Observer, the Stern report says unchecked climate change would cost up to 3.68 trillion pounds (5.48 trillion euros, 6.98 trillion dollars) -- more than World Wars I and II and the Great Depression of the 1930s.

It also warns that the world needs to spend about one percent of global gross domestic product -- equivalent to about 184 billion pounds -- on the issue now or face a bill up to 20 times higher than that in future, the paper says.

Stern also calls for a successor to the Kyoto agreement on greenhouse gases to be signed next year, not in 2010 or 2011 as planned, because the problem is so urgent, it adds.

Failure to act quickly would trigger a global recession, he reportedly adds, and calls for an international framework to tackle the issue.

The Observer says his report is the first heavyweight contribution to the debate on climate change by an economist rather than a scientist.

Environmental activist group Greenpeace said it removed any doubt about the need to tackle climate change.

"If we are to avert catastrophe then there has to be a real cost to emitting carbon and that means higher taxes on flying and gas-guzzlers. We owe it to future generations," a spokesman said.

Commenting on Stern's findings, environment secretary David Miliband quoted scientists as saying that action needed to be taken within 15 years to change the way energy was produced.

"I think it is very significant that the economics revealed by Sir Nicholas Stern's report should be that the longer we wait, and certainly the longer we wait beyond the 10-15 year timeframe that is set by the scientists, the more costly it will be," he told Sky News television.

Miliband added that discussions were "going on inside government" about green taxes but did not comment on a Mail on Sunday newspaper report that ministers had drawn up plans for taxes on fuel, cars, air travel and consumer goods to fight global warming.

Environmental issues were pushed up the political agenda in Britain last year by the election of David Cameron as leader of the main opposition Conservative party on a green platform.

Under him, the Tories have enjoyed a resurgence of public support, with many opinion polls putting the party ahead of Prime Minister Tony Blair's ruling Labour.

Cameron has so far avoided committing himself to detailed policies on the environment but indicated in a BBC television interview Sunday that he might tax air travel if he came to power.

He also said he would install wind turbines and solar panels at 10 Downing Street, the prime ministerial residence, if he wins the next general election.

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