A "bogus accounting trick" has been used to justify building a third runway at Heathrow and ministers have seriously underestimated the environmental impact of the development, it was claimed yesterday.
Campaigners and opposition MPs warned that a cost analysis of the project's impact on the climate came up with a figure which was only a third of that set out in a global warming study for the Government by the former Treasury official Sir Nicholas Stern.
The consultation paper on the expansion of Heathrow, published last month, said the cost of climate change caused by the project would be 4.8bn. But Friends of the Earth insisted that the true figure, based on Sir Nicholas's estimate, was 13.4bn.
The Department for Transport claims building a third runway will increase the number of flights to and from Heathrow from 473,000 a year to 700,000-plus by 2030, and generate an extra 181 million tonnes of carbon dioxide emissions by 2080.
Simon Bullock, of Friends of the Earth, said: "The Government is using a bogus accounting trick to force through its massively damaging Heathrow expansion. The economic and social damage from climate change far outweighs the benefits to rich passengers 30 years from now getting slightly cheaper flights.
"If the Government is as serious as it claims to be about tackling climate change, it would announce a halt to new runways, at Heathrow and elsewhere."
Martin Horwood, the Liberal Democrat environment spokesman, added: "This is a disgrace. The Government has massaged the figures to hide the impact of this unwanted and unnecessary expansion. As a result, it has given the green light to one of the very projects that will stop it meeting its own carbon reduction targets. The cost to the environment and the economy will be dreadful."
The consultation claims the net benefit of the scheme will be 5bn, "even after taking account of climate change and noise costs". But Friends of the Earth said that if ministers used a less optimistic figure for the cost of climate change, any economic benefits of the runway would be wiped out by its environmental consequences. Guidance from the Department for Environment, Food and Rural Affairs, published in July, estimated that the cost of Heathrow's extra C02 emissions would be 19 per tonne at 2000 prices. The Stern review, by contrast, quoted 53 per tonne, based on the price of carbon if the world did not curb emissions, Mr Bullock said.
The public has until 27 February to respond to the proposals for a third runway, a sixth terminal and changes to take-off and landing patterns and aircraft approach routes.
Edward Lister, the leader of Wandsworth Council and a spokesman for the 2M group, which represents 12 local authorities affected by the development, said yesterday: "The assumptions in the consultation are totally flawed. This whole thing will end up in court."