Heathrow Airport’s plans to build a third runway have been thrown into doubt after a court ruling.
The Court of Appeal said the government’s decision to give the go-ahead to Heathrow’s expansion plans was unlawful.
The court said the government would not appeal against the judgement.
But the judges said that in future, a third runway could go ahead, as long as it fits with the UK’s climate commitments.
The case was brought by environmental groups, councils and the Mayor of London.
The Court of Appeal found that the government had not followed UK policy when backing the controversial expansion plans.
It said that the government had a duty to take into account the Paris climate agreement, which seeks to limit global warming.
It was “legally fatal” to the government’s Heathrow expansion policy that it did not take those climate commitments into account, the judges said.
One of the environmental groups which brought the case, Greenpeace, said the government needed to “permanently ground Heathrow’s expansion plans”.
Greenpeace UK’s executive director, John Sauven, said: “The third runway is already on its knees over costs, noise, air pollution, habitat loss and lack of access, and now Heathrow Ltd has yet another impossibly high hurdle to clear.
“No amount of spin from Heathrow’s PR machine can obscure the carbon logic of a new runway. Their plans would pollute as much as a small country. Boris Johnson should now put Heathrow out of its misery and cancel the third runway once and for all. No ifs, no buts, no lies, no U-turns.”