700,000 flights a year could go through Heathrow by 2030 if Government give plans the green light in the coming weeks.
London's Mayor has pledged to give £15,000 to a group planning to mount a legal challenge against a third runway at Heathrow.
Boris Johnson is contributing to the money being raised for possible court proceedings by the 2M Group - a coalition of more than 20 local authorities in Greater London and Berkshire.
The group says it is concerned about the impact that increased air and noise pollution from another runway will have on its four million residents.
Transport secretary Geoff Hoon is expected to approve BAA's plans for a third runway next month.
It is expected to be built by 2030, expanding the number of flights going through the airport from 480,000 to 700,000 a year.
A spokesperson for the Mayor told edie that Mr Johnson will support a legal challenge if it is shown that the proper decision making process has not been followed by Government.
She said: "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.
"He is also keen that a thorough examination of the long-delayed results of the Government's consultation and environmental tests takes place."
Mr Johnson has previously said he is opposed to the expansion of Heathrow because of the likely impact of noise and air pollution.
Members of the 2M Group took their arguments to Edinburgh on Tuesday, as the Scottish Parliament held an inquiry into the possibility of a new high speed rail network linking England and Scotland.
Wandsworth Council leader Edward Lister, who was part of the delegation, said: "Any government faced with decisions on expanding airports in the South East should be making a comparison with the potential economic benefits to the rest of the UK from investment in high speed rail and the savings in carbon emissions.
"It's not just the extra aircraft noise that will be so damaging for people living under the Heathrow flight path, it's the added congestion on local roads and worsening air pollution."
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