The Court of Appeal will today decide if expansion at Heathrow is in breach of climate change commitments.
Environmentalists are hoping the court will rule that plans to build a third runway at the hub failed to take into account climate change commitments.
If the government loses, it could re-start the appraisal process but it would have to make the case that it was compatible with fighting climate change.
Update: Court rules third runway at Heathrow unlawful
Opponents who have brought the case include local residents, councils, the mayor of London, and environmental groups including Greenpeace.
Former transport secretary Chris Grayling approved a third runway in April 2018 despite the government’s climate change committee saying it was not compatible with a climate neutral economy.
Activists say Grayling should have taken into account the Paris deal on climate change, which pledged to limit global warming to 1.5 degrees if possible.
Before becoming prime minister Boris Johnson vowed to lie down in front of the bulldozers to prevent the expansion of Heathrow.
The government is likely to announce within hours of the decision whether or not to accept the ruling or appeal.