Campaigners who oppose the construction of a third runway at London’s Heathrow Airport won a landmark Court of Appeal victory today when the project was halted on climate change grounds.

The court’s decision that the government should have considered its commitments under the Paris Agreement when making the decision to approve the controversial £14bn project to expand Heathrow has put the entire scheme in doubt.

It is one of the most high-profile examples to date of climate change considerations being deployed in court challenges successfully and follows December’s decision by the Dutch Supreme Court to order the government to accelerate the country's emissions-reduction timetable in the Urgenda case.

The Heathrow hearings have featured an array of appellants and interested parties and seen several top law firms line up against and alongside each other including Bryan Cave Leighton Paisner, which acted for Heathrow Airport, CMS UK, which acted for Arora Holdings - a major landowner at the airport, and DAC Beachcroft, which acted for Heathrow Hub, which has put forward alternative proposals.

In a summary of their judgments, Lord Justice Lindblom, Lord Justice Singh and Lord Justice Haddon-Cave said the fact a government policy statement on airport expansion – the Airports National Policy Statement (ANPS) - did not take account of commitments made under The Paris Agreement was “legally fatal”.

They added: “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.”

Rowan Smith, a solicitor in the environmental law team at UK firm Leigh Day, which acted for key challenger Friends of the Earth, said:  "The Lord Justices simply followed the legal framework set by parliament, citing as they did their fundamental function to maintain the rule of law, and found that the Paris Agreement was so obviously material to a decision on Heathrow Airport expansion that the secretary of state was legally bound to consider it. The government must now go back to the drawing board and come up with a new policy, which lawfully fulfils its sustainable development duties and protects future generations.”

Meanwhile, the case further highlights tension between the UK courts and the government following the Supreme Court’s historic ruling in September to strike down Prime Minister Boris Johnson’s suspension of parliament. 

“For 17 years politicians have been debating Heathrow expansion,” Tweeted barrister Jon Holbrook, a ‘Brexiteer’. “Yet their conclusion has been trumped by three judges. A country where law is master and politics its servant isn’t a democracy.”

However, barrister Alan Bates, who acted for the Government Legal Department, responded to one Tweet by arguing it was “unhelpful and wrong... to. describe the Court’s decision as ‘insane’”. 

He added: “The Court has sought to apply the law, not to decide whether Heathrow expansion is a good thing. (I point this out despite being one of the barristers resisting the challenges - which mostly failed).”

Heathrow immediately said it would appeal the judgment although the government has indicated it will not mount a challenge.

Earlier this month, the International Bar Association (IBA) unveiled a model statute to help citizens launch legal challenges against their governments for failing to take action over climate change.