Lord Justice Lindblom ruled that the former transport secretary Chris Grayling's failure to take into account the Paris Agreement was "legally fatal" to the Airports National Policy Statement (ANPS) he approved in June 2018.
The ruling means that the current secretary of state, Grant Shapps, must now review the ANPS - which confirmed a third Heathrow runway as the government's preferred option - in the light of the court's decision. Until that is done, the ANPS will have no legal effect.
The ANPS says that it would be used by the secretary of state "as the primary basis for making decisions on any development consent application for a new Northwest Runway at Heathrow Airport, which is the government’s preferred scheme".
It adds that it would also be a "relevant consideration in respect of applications for new runway capacity and other airport infrastructure in London and the South East of England".
Delivering a stunning victory to environmental pressure groups, Friends of the Earth and Plan B, the judge said: "We have concluded that the challenges should succeed in one important respect.
"This relates to the legislative provisions concerning the government’s policy and commitments on climate change, in particular the provision in section 5(8) of the Planning Act 2008, which requires that the reasons for the policy set out in the ANPS 'must … include an explanation of how the policy set out in the statement takes account of government policy relating to the mitigation of, and adaptation to, climate change'.
"We have concluded, in particular, that the designation of the ANPS was unlawful by reason of a failure to take into account the government’s commitment to the provisions of the Paris Agreement on climate change, concluded in December 2015 and ratified by the United Kingdom in November 2016.
"We have concluded that the ANPS was not produced as the law requires, and indeed as Parliament has expressly provided. The statutory regime for the formulation of a national policy statement, which Parliament put in place in the Planning Act, was not fully complied with.
"The Paris Agreement ought to have been taken into account by the secretary of state in the preparation of the ANPS and an explanation given as to how it was taken into account, but it was not. That, in our view, is legally fatal to the ANPS in its present form."
The judge, who was sitting with Lords Justice Singh and Haddon-Cave, added: "The court will not permit unlawful action by a public body to stand. Appropriate relief must therefore be granted.
"We have concluded that the appropriate remedy is a declaration, the effect of which will be to declare the designation decision unlawful and to prevent the ANPS from having any legal effect unless and until the secretary of state has undertaken a review of it in accordance with the relevant statutory provisions, including the provisions of section 6, 7 and 9 of the Planning Act 2008. Any such review would have to be conducted in accordance with the judgment of this court."
But the judge also told the court: "Our decision should be properly understood. We have not decided, and could not decide, that there will be no third runway at Heathrow.
"We have not found that a national policy statement supporting this project is necessarily incompatible with the United Kingdom’s commitment to reducing carbon emissions and mitigating climate change under the Paris Agreement, or with any other policy the government may adopt or international obligation it may undertake.
"The consequence of our decision is that the government will now have the opportunity to reconsider the ANPS in accordance with the clear statutory requirements that Parliament has imposed."
The judge added that the government had not sought permission to appeal to the Supreme Court against the court's decision.