Boris Johnson, the mayor of London, has threatened to take legal action against the government if ministers go ahead with plans to build a third runway at Heathrow.

He and a coalition of local authorities representing 4m people believe expansion, which would increase the number of flights at Heathrow from 480,000 to 700,000 a year, would breach European Union laws on pollution.

Legal action could derail Geoff Hoon’s intention to ensure the new runway is operational by 2030. He is expected to give the go-ahead to the expansion next month.

A spokesman for Johnson said City Hall and the 2M group of local authorities would analyse the government’s decision to determine whether there was a basis for legal action.

Johnson has pledged an initial £15,000 towards the costs of mounting a legal challenge if lawyers give the go-ahead, and may pay more if needed. His spokesman said the business case for the runway expansion did not outweigh “concerns for the local environment in terms of noise and air quality”.

The case for a potential judicial review is likely to rest on whether EU air pollution limits would be breached as a result of the expansion.

The government white paper said approval would be given only if the runway met noise and environmental standards. But critics, including the Environment Agency, say the extra nitrogen dioxide emissions produced by expansion will breach EU limits, which come into force in 2010. Hilary Benn, the environment secretary, has confirmed that the government will seek permission to postpone compliance until 2015.

Stavros Dimas, the European commissioner for the environment, has vowed to take enforcement action if the limits are breached.

The warning came after it emerged that Department for Transport (DfT) officials doubted their own proposed measures for reducing pollution at the airport. The DfT’s consultation paper on the Heathrow expansion hid concerns, revealed under freedom of information laws, that measures to mitigate pollution are likely to be “too costly or impractical . . . or politically unacceptable”.

The DfT’s September 2007 “risk register” for Heathrow expansion also shows that officials assessed as “high” the risk that mitigation measures might not be deliverable.

Edward Lister, leader of Wandsworth council and speaking on behalf of the 2M group, said: “We are trying to show that the whole basis on which the government wants to justify expansion is bogus.

“It’s great that Boris is supporting this action and it’s not too late for the government to think again.”

Opponents of Heathrow’s expansion have been emboldened by the impact of the recession, which is reducing demand for air travel. Consultants for WWF UK, the environmental group, predict that higher oil prices mean demand in 2030 will have grown by 38% less than is assumed by the DfT.

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