Greenpeace has formed an "alliance" with four local authorities to prepare a legal challenge against Heathrow expansion.
The organisation announced it would share costs and provide "new environmental expertise" to the partnership with Hillingdon, Richmond, Wandsworth and Windsor and Maidenhead councils.
More groups could join the group in the coming days as the wait continues for the Government to choose whether to expand airport capacity at Heathrow or Gatwick.
Transport Secretary Chris Grayling, who will make the much-anticipated announcement once a decision has been reached, told MPs on Monday that he was optimistic the project would survive any attempts to slow it down in the courts.
Asked by the Commons' Transport Select Committee what steps will be taken to ensure expansion is not "boggged down" by a "decade of legal challenges", Grayling replied: "It's very much my hope and belief that, although there will undoubtedly be opposition to whatever we do, the will of Parliament, the democratically-elected parliament of our nation, is what will ultimately count."
-- Charlotte Lomas (@LomasChar) October 17, 2016
The campaigners preparing a legal challenge worked together in 2010 to win a High Court battle over the plans of Gordon Brown's government to build a third runway at Heathrow.
The group says the latest expansion scheme at the west London hub will have more severe environmental impacts than the proposal at the start of the decade.
It argues that fresh evidence on the effects of air and noise pollution make the new scheme less likely to pass judicial review.
Leader of Wandsworth Council Ravi Govindia warned Prime Minister Theresa May that she should be in "no doubt" about the scale of opposition the Heathrow expansion will face.
-- Wandsworth Council (@wandbc) September 15, 2016
He added: "A scheme this environmentally offensive will unite a force of opposition no government can overcome. It's wrong on every level, legally undeliverable and will end in failure after years of wasted effort."
Greenpeace UK executive director John Sauven said: "A third runway at Heathrow would be an air pollution and carbon time bomb.
"It would jeopardise the Government's chances of meeting legally-binding air pollution and climate targets designed to protect the health and security of millions of people. If ministers are willing to trample over these fundamental laws, we're ready to take them to court to stop them."
On Heathrow - @greenpeace are backing four councils who SUPPORT Gatwick expansion. Greenpeace is 100% against airport expansion, right?
-- Back Heathrow (@BackHeathrow) October 17, 2016
Downing Street has insisted the Government remains committed to making a decision on expansion amid reports of continued wrangling among ministers.
Number 10 refused to be drawn on speculation that May had ordered a "short pause" to give ministers a chance to air their views before a final decision is taken.
The BBC said the Cabinet would discuss the issue when it met on Tuesday but the final decision would be left to the Economic Affairs (Transport) sub-committee, chaired by Mrs May, at a meeting later this month.
Airport expansion has split the Cabinet with Foreign Secretary Boris Johnson and Education Secretary Justine Greening opposed to a new runway at Heathrow - widely seen as the most likely outcome.
A decision was repeatedly delayed by May's predecessor David Cameron and Tories may be offered a free vote, or opponents given a leave of absence from the Commons when the issue comes up.