The Government's blueprint for tackling climate change could cost five times more than expected, a leaked United Nations report shows today.
The report comes as Lord Lawson, the Conservative former chancellor, launches a scathing attack on "eco-fundamentalism" and says a costly new carbon tax could wreck the economy.
The Stern review, commissioned by Gordon Brown, outlined plans this week to combat global warming by capping greenhouse gas emissions.
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It raised fears of floods from rising sea levels, water shortages from melting glaciers and extreme weather potentially causing up to 200 million people to flee their homes.
Sir Nicholas Stern, head of the government economic service, estimated that the cost of keeping emissions at 550 parts per million was likely to be "around one per cent of GDP by 2050".
He also said that the cost of not acting to combat climate change could be between five and 20 per cent of global GDP.
The calculations were used to justify Sir Nicholas's calls for immediate action and extra taxes
But a leaked draft UN report, quoted in today's edition of The Business, casts doubt on the basis for his conclusions.
A report by the authoritative UN Intergovernmental Panel on Climate Change (IPCC), due for publication next year, warns that the cost of achieving the same goal would be between "one per cent and five per cent loss of global GDP".
The less ambitious target of stabilising emissions at 650ppm would cost less than two per cent of GDP, according to extracts from the UN report. Sir Nicholas has acknowledged that his review was partly based on the IPCC's 2001 report.
Business: Page B3
The Stern Review of the Economics of Climate Change put the cost of keeping emissions at 550 parts per million at one percent of global GDP

A leaked draft report, by the UN Intergovernmental Panel on Climate Change (IPCC), due to be published next year, estimates this cost could be between one and five percent of global GDP.
 
The Stern Review put the costs of not acting to resist climate change could be between 5 and 20 percent GDP.

It is reported that UK Ministers are to travel to India and America to promote the Stern Review. Their task will not be eased by the leaked IPCC report which is now said to be available freely on the internet.

Following the Stern Review, Chancellor Gordon Brown urged the member states of the European Union Emissions Trading Scheme to tighten their targets even further.