The Mayor of London has promised to help finance a legal battle against the Government if it decides to go ahead with the expansion of Heathrow Airport.

Boris Johnson has given his support to the 2M group - which is made up of 21 councils and represents a combined population of 4m people - and campaigners who are fighting against a third runway.

Transport Secretary Geoff Hoon is expected to make a decision on the airport's proposed third runway next month.

Mr Johnson has pledged £15,000 for any legal bid but said further funding would have to be discussed after the Government had made its decision.

John Stewart, chairman of campaign group HACAN, said a decision to go to court will not be taken until the organisation has heard what the Government has decided.

He said: "We have been in talks with lawyers, along with the local authorities and the Mayor's office.

"If the Government gives the green light to the expansion then we may proceed with legal action.

"We believe we have a strong case for a challenge."

He said they were really pleased with Mr Johnson's support of the group's campaign, which is "a real boost for us".

Councillor Ray Puddifoot, leader of Hillingdon Council, welcomed the Mayor's support. He said: "The support of Boris is very good, because although the money is helpful, what's more important is the symbolism that he is willing to fight this with us."