Hillingdon and Wandsworth Councils have made an alliance with Greenpeace UK over Heathrow Airport.
Richmond, and Windsor and Maidenhead Councils are also involved in the partnership, which aims to prepare grounds for a joint legal challenge against Heathrow expansion.
An announcement was expected this week, but the BBC reports it has been put off to allow a Cabinet meeting to discuss the decision, so ministers can make their comments.
Greenpeace and the four local authorities say both Heathrow expansion schemes would be unlawful due to their environmental impacts, the aircraft noise footprint, and the impact on local transport.
The councils began work on a legal case last year, Greenpeace will now share costs.
The group say today’s Heathrow expansion scheme has more severe environmental impacts than the 2010 proposal, approved by the Labour Government, but overturned by the Conservatives. Heathrow say that their expansion plans will comply with all legislation.
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.”