Campaigners trying to block Heathrow expansion will find out if their Court of Appeal challenge against controversial plans for a third runway has succeeded.
A group of councils in London - including Richmond and Wandsworth - affected by the expansion, environmental charities including Greenpeace, Friends Of The Earth and Plan B, and Mayor of London Sadiq Khan, brought legal action over the Government's approval of the plans.
Leading judges will give their ruling at 10am on Thursday, following a hearing in October last year.
Lawyers for the campaigners previously told the court that, when considering the proposals, the then transport secretary Chris Grayling did not take enough account of environmental legislation or of climate change issues.
Prime Minister Boris Johnson opposed the expansion of the west London airport when he was London's mayor and promised to "lie down ... in front of those bulldozers" to stop the runway being built.
Earlier this month he said there was "no immediate prospect" of construction beginning.
Bristol Airport's bid to increase its annual passenger capacity from 10 million to 12 million was recently rejected by councillors due to environmental concerns.
In June last year, the Welsh Government scrapped a long-proposed £1.4 billion M4 relief road because of its cost and impact on the environment.
Anti-Heathrow expansion campaigners appealed against a High Court ruling in May, which rejected four separate judicial reviews of the Government's decision to approve the plans.
They say the Government's National Policy Statement (NPS) setting out its support for the project failed to properly deal with the impact on air quality, climate change, noise and congestion.
Government lawyers urged the judges to reject the appeals, arguing that the High Court reached the correct conclusion.
The appeals were heard alongside a separate challenge by Heathrow Hub, a consortium with a rival proposal for the airport's expansion.
Support from Labour MPs helped push through the proposals to expand Europe's busiest airport with an overwhelming majority of 296 in a Commons vote in June last year.
Mr Grayling said at the time that the new runway would set a "clear path to our future as a global nation in the post-Brexit world".
Heathrow says it could open the third runway "between early 2028 and late 2029".Its previous target date was 2026.
Heathrow said the delay was due to the Civil Aviation Authority, which refused to approve its bid to nearly quadruple the amount it spends before obtaining final planning permission.
Campaigners are expected to gather outside court before the ruling, from 9am.