More claimants could join the alliance in the coming days as media reports have suggested a final decision has now been delayed until next week.
Ministers have refused to rule out Heathrow from their airport development shortlist, leading the campaign group to prepare grounds for judicial review.
Greenpeace and the four local authorities say both Heathrow expansion schemes would be unlawful due to their unrivaled environmental impacts, which include exacerbating illegal levels of air pollution, increasing Europe’s worst aircraft noise footprint and stretching the local transport network beyond breaking point.
The councils jointly instructed Harrison Grant Solicitors to prepare their legal strategy last year and Greenpeace will now share costs and bring new environmental expertise to the partnership.
The campaigners also worked together back in 2010 to successfully overturn the Brown Government’s backing for a third runway in the High Court. Later that year the scheme was emphatically ruled out by the incoming Cameron Government.
The group say today’s Heathrow expansion scheme is even bigger and has more severe environmental impacts than the 2010 proposal, and will fail the same legal tests. They argue that new evidence on the severe health impacts of air and noise pollution make the new scheme far less likely to pass judicial review.
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.
"Theresa May has made much of her determination to help ordinary people and hold corporations to account. The decision on airport expansion will be the acid test of whether she's willing to deliver on that promise."
Leader of Wandsworth Council Ravi Govindia said:
“The Prime Minister should now be in no doubt about the scale of opposition Heathrow expansion will face. 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. We once again urge the Government to accept the inevitable and rule out Heathrow.”
Leader of Hillingdon Council Ray Puddifoot said:
“A new runway at Heathrow will make already illegal air pollution levels around the airport worse and give Heathrow a noise pollution record that is worse than the top five European airports put together. We urge the Government to consider Gatwick as the most realistic and sustainable option, and one which can be delivered at a much reduced cost to the taxpayer and with far less damage to the environment and wellbeing of people."
Leader of Richmond Council Lord True said:
“Let no-one ever call our residents nimbys. Day in day out they put up with the worst noise and air quality pollution in Europe from before dawn to midnight. But enough is enough. A Bigger Heathrow is wrong not just for environmental reasons, but wrong for competition, wrong for security from terrorism and wrong because it represents the very opposite of what this new government says it stands for – the victory of the privileged few, international monopolistic investors, over the lives of ordinary families. It will never be built”.
Leader of the Royal Borough of Windsor and Maidenhead Council Simon Dudley said:
"A third runway at Heathrow isn't a deliverable alternative so isn't a realistic choice. As the number of claimants in our judicial review group increases, so does the coalition of opposition and available financial resources to stop a third runway at Heathrow, and protect our residents".