A third runway at Heathrow would lead to "unavoidable" rises in air pollution, noise and traffic jams, the new head of the Environment Agency warned today.

In a big blow to the Government, former Labour Culture Secretary Lord Smith of Finsbury said approving the airport expansion would be "a mistake".

Gordon Brown and Transport Secretary Ruth Kelly have both promised that the environment will be a central consideration in any decision on Heathrow, expected later this year.

But Lord Smith, who was recently appointed as chairman of the Environment Agency, made clear that its own experts had concluded there was no green case that could be made for the proposals.

He dismissed the Department of Transport's insistence that a new runway at Heathrow Airport could be environmentally sustainable.

"The increases in volume of air traffic and the consequent increases in congestion on the ground are, from the analysis that we've done, pretty unavoidable," he told The Independent. "I think the Government is making a mistake and I will carry on telling them that I think they are making a mistake."

His criticism came as the Competition-Commission prepared to release its long-awaited report on London's airports, with the likely conclusion that BAA's monopoly over Heathrow, Gatwick and Stansted should be broken up. An extra runway at Gatwick may be proposed in a bid to boost competition.

Lord Smith also said that other key areas of the environment were not being treated seriously enough and set himself firmly against proposals to build a new coal-fired power station at Kingsnorth in Kent.

He opposed the Kingsnorth plant - with others to follow - because he was not satisfied the promised technology to "capture and store" carbon would have been developed in time for its planned opening next decade. "My view would be that we should not go ahead with the development of new coal-fired generation unless those [clean coal] technologies are in place."

Lord Smith also said he was dismayed that proposals to give planning power over major building projects to a national body would not require it to consider the environmental impact. He said he hoped the Government was only guilty of a "sin of omission" and would amend the forthcoming Environment Bill.

The former minister also warned that plans will soon have to be drawn up to evacuate people from coastal stretches that are being eroded by global warming.

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