Plans for a third runway at Heathrow have been thrown into doubt following a decision by the court of appeal which ruled that the project was illegal because ministers did not adequately consider the government’s climate change commitments in line with the Paris agreement.

The decision is potentially a massive blow to the Heathrow expansion project which the construction sector has been looking forward to receiving the go-ahead. Prime minister Boris Johnson has previously indicated that the government would not seek to overturn the court’s decision if it ruled against the project in a case which was brought by the legal charity Plan B. Johnson also famously said in 2015, when an MP, that he would “lie down in front of those bulldozers and stop the construction”.

Reacting to the court of appeal’s ruling, Plan B’s Tim Crosland said: “The court ruling is bad news for all businesses and investors in the carbon economy, who will have to factor in the increasing risks of legal challenges . . . but really it is good news for everyone, since all of us – including businesses and investors – depend on maintaining the conditions which keep the planet habitable.”

The appeal court said that the government would not appeal the judgement. Boris Johnson might be minded to use the ruling to abandon the project, which would provoke an outcry from the construction industry and the business community or they could seek a fresh appeal in the supreme court to have the ruling overturned or it could start again by drawing up a new policy document that gave full consideration to the Paris agreement commitments in a bid to get the runway approved.

Whatever it decides, the government is certain to come under political and business pressure in the aftermath of this judgement which will have far-reaching implications for all major projects and their effect on the natural environment. This effect could well be global as, for the first time, a court has confirmed that the Paris agreement temperature goal has binding effect.

Heathrow Airport reacted to the news by pointing out that the court of appeal had dismissed all appeals against the government, including on noise and air quality, "apart from one which is eminently fixable,” a spokesperson said. 

"We will appeal to the supreme court on this one issue and are confident that we will be successful," they added. "In the meantime, we are ready to work with the government to fix the issue that the court has raised. Heathrow has taken a lead in getting the UK aviation sector to commit to a plan to get to net zero emissions by 2050, in line with the Paris Accord. Expanding Heathrow, Britain’s biggest port and only hub, is essential to achieving the prime minister’s vision of global Britain.  We will get it done the right way, without jeopardising the planet’s future. Let’s get Heathrow done," the spoksperson added.