Boris Johnson has pledged an initial £15,000 towards the cost of mounting a legal challenge if ministers go ahead with plans to build a third runway at Heathrow, and may pay more if needed.

The mayor of London and a coalition of 21 local authorities representing 4 million people believe the expansion of Heathrow would breach EU laws on pollution.

The transport secretary, Geoff Hoon, is expected to give the green light to the scheme next month, which would see a new runway in place by 2030 and expand the number of flights at Heathrow from 480,000 to 700,000 a year.

A spokesman for Johnson said that the mayor and the 2M group of local authorities against Heathrow expansion would analyse the government's decision to determine whether there was a basis for legal action.

The spokesman said: "As the representative of every Londoner, the mayor has agreed to make a contribution to the funds 2M is raising to cover the possible costs of a legal challenge."

The 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 mayor is anxious to ensure that the concerns of Londoners whose environment would be affected by a further expansion of Heathrow are fully addressed," the spokesman said.

"He is also keen that a thorough examination of the long-delayed results of the government's consultation and environmental tests takes place."

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.

The 2M leaders have already been to Strasbourg to brief the EU's environment commissioner, Stavros Dimas, on the matter.

Hilary Benn, the environment secretary, has confirmed that the government will seek permission to postpone compliance until 2015.

The Conservative party nationally is vehemently against the third runway.

Theresa Villiers, the shadow transport secretary, has gone as far as to warn companies not sign any contracts related to building the runway.

The Tories instead want to link Heathrow up to a high-speed rail network, leading to Birmingham, Leeds and Manchester.

Villiers has also poured cold water on her fellow Tory Johnson's plans for a new airport in the Thames Estuary, however.

Johnson has set Doug Oakervee, an experienced engineer and executive chair of Crossrail, the task of leading the Greater London authority's preliminary feasibility study into a new site to accommodate growing demand for flights.