The Government has triggered the next step towards Heathrow expansion in the wake of its victory in a Commons vote on the controversial plans.
Transport Secretary Chris Grayling has formally designated the Airports National Policy Statement (NPS), paving the way for the airport to submit a formal planning application.
This means a group of councils and campaigners have six weeks to submit a legal challenge to block the project.
A judicial review against the decision is being launched by four London local authorities affected by the expansion - Wandsworth, Richmond, Hillingdon and Hammersmith and Fulham - in partnership with Greenpeace and mayor Sadiq Khan.
Greenpeace UK executive director John Sauven said: "If ministers don't want to uphold the laws protecting us from toxic fumes and climate change, we're going to ask a court to do that."
"I have always been clear that this issue goes beyond party politics and this result demonstrates the clear desire to get on with delivering this vital scheme.
"There is still much to be done, including defending this decision against potential legal challenges, but we are absolutely committed to working closely with local communities and ensuring Heathrow stick to their promises on addressing the local and environmental impacts of expansion."
The plans have been met by pockets of strong opposition within both Labour and the Conservatives.
Former Northern Ireland Secretary Theresa Villiers was one of eight Conservative MPs to rebel against the Government, as well as Greg Hands, who quit as a minister ahead of the vote so he could oppose the plans.
Boris Johnson, a long-time opponent of expansion, missed the vote as he was in Afghanistan.
Following Tuesday's designation of the NPS, Heathrow will submit detailed planning to the Planning Inspectorate.
This will involve the airport conducting a further consultation with affected communities on the finer details of their scheme and the associated compensation packages and mitigation measures.
The Department for Transport said Heathrow will keep Mr Grayling updated with its progress over how it will meet commitments to the environment, communities, domestic connectivity and building a new runway in a timely and cost effective manner.
Construction could begin in 2021, and the third runway operational by 2026.