A court has ruled the UK government’s decision to build a third runway at Heathrow was unlawful.

Environmentalists brought the case to court claiming the decision failed to take into account climate change commitments.

The government will not appeal the decision but Heathrow airport has said it will.

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Former transport secretary Chris Grayling approved a third runway in April 2018 despite the government’s climate change committee saying it was not compatible with a climate neutral economy.

Activists say Grayling should have taken into account the Paris deal on climate change, which pledged to limit global warming to 1.5 degrees if possible.

Aviation Environment Federation deputy director Cait Hewitt welcomed the news.

“This is a huge win for the climate, and leaves Heathrow’s third runway plans in tatters,” she said. “In presenting plans for a third runway to MPs, the government failed, the court has found, to assess whether this was compatible with the Paris Agreement.

“This ruling should mark the end of plans for any new runways in the UK. The government should stand up to the airports lobby, drop its support for airport expansion, and invest instead in low-carbon transport and supporting British tourism.”

Jock Lowe from Heathrow Hub, which is in favour of extending the existing northern runway, said: “This is our ‘we told you so’ moment.

“There was always a huge risk that the unnecessarily complicated and expensive third runway would fail on environmental grounds and that Boris Johnson would stop it, and so it has proved.”

But Heathrow airport said the court’s decision does not prevent expansion going ahead in future.

“We will appeal to the Supreme Court on this one issue and are confident that we will be successful. In the meantime, we are ready to work with the government to fix the issue that the court has raised.”

British pilots have also urged the government to do all it can to get airport expansion plans back on track.

The British Airline Pilots Association (Balpa) says that delays to expansion will increase congestion and slow down trade.

General secretary Brian Strutton said: “We understand the challenge that the aviation industry faces when it comes to climate change but also recognise the vital contribution to the UK economy that the industry makes.

“Without expansion we will see further congestion, stagnation in the sector and will struggle to keep pace with global competitors.

“We urge the Government to ensure that Heathrow expansion fits within its Paris climate obligations and does so as quickly as possible. We need to get this done.”