Heathrow Airport’s controversial third runway could be in jeopardy after being ruled illegal by the Court of Appeal, whose judges found in favour of environmental protesters opposing the plan.

The case was brought by a group of councils in London affected by the expansion; environmental charities including Greenpeace, Friends Of The Earth and Plan B; and mayor of London Sadiq Khan.

Khan said on Thursday morning: “We won! Today we blocked the Tory government plans to build a third runway at Heathrow Airport.

“Today’s judgment is a major victory for all Londoners who are passionate about tackling the climate emergency and cleaning up our air.”

Will Rundle, head of legal at Friends of the Earth, added: “This ruling is an absolutely ground-breaking result for climate justice. We were fighting a project that would have had dire implications for present and future generations.” 

But Heathrow Airport said it will appeal to the Supreme Court.

A spokeswoman said: “The Court of Appeal dismissed all appeals against the government – including on noise and air quality – apart from one which is eminently fixable.

“We will appeal to the Supreme Court on this one issue and are confident that we will be successful.

The government has yet to make a statement on the decision but Greenpeace urged prime minister Boris Johnson to formally cancel the project.

Its executive director John Sauven said: “We still need the government to permanently ground Heathrow’s expansion plans.

“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.”

Will Rundle from Friends of the Earth said the ruling”goes to show the massive importance of the legal system to check the clear abuse of state power by government, such as in this case.

“Shockingly, this case revealed that the government accepted legal advice that it should not consider the Paris Agreement when giving the third runway the go-ahead. The Court has said very clearly that was illegal.

“This judgment has exciting wider implications for keeping climate change at the heart of all planning decisions. It’s time for developers and public authorities to be held to account when it comes to the climate impact of their damaging developments.”

The claimants had argued climate change commitments were not taken into consideration when the expansion plans were formulated.

Giving their ruling on Thursday, Lords Justice Lindblom, Singh and Haddon-Cave said the government did not take enough account of its commitment to the Paris Agreement on climate change when setting out its support for the proposals in its National Policy Statement (NPS).

Lord Justice Lindblom told the court: “The Paris Agreement ought to have been taken into account by the secretary of state in the preparation of the NPS and an explanation given as to how it was taken into account, but it was not.”

Former transport secretary Chris Grayling had given the third runway the go-ahead in April 2018, despite warnings from the government’s climate change committee that expanding the airport was not compatible with a climate neutral economy.

Green groups argued that Grayling should have taken the Paris deal, which pledges to limit global warming to 1.5C, into account before making the decision.   

Boris Johnson missed the Commons vote on the expansion, as he was in Afghanistan in his role as foreign secretary. 

Britain has committed to reaching net zero emissions by 2050.

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