Heathrow Airport’s plans to increase capacity of Europe’s biggest travel hub by over 50% were stalled Thursday when a British court said the government failed to consider its commitment to combat climate change when it approved the project.

The ruling throws in doubt the future of the 14 billion-pound ($18 billion) plan to build a third runway at Heathrow, the west London hub that already handles more than 1,300 flights a day.

While Heathrow officials said they planned to appeal, Prime Minister Boris Johnson’s government indicated it wouldn’t challenge the ruling by the Court of Appeal.

“We won!” said London Mayor Sadiq Khan, a long-time opponent of the project who joined other local officials and environmental groups in challenging the national government’s approval of Heathrow’s expansion plans.

At stake is a project that business groups and Heathrow officials argue is crucial for the British economy as the U.K. looks to increase links with countries from China to the United States after leaving the European Union. Heathrow has already reached the capacity of its current facilities, and a third runway is needed to serve the growing demands of travelers and international trade, they say.

Environmental campaigners, however, challenged the project because of concerns that a third runway would encourage increased air travel and the carbon emissions blamed for global warming. The British government has committed to reducing greenhouse gas emissions as a signatory to the 2016 Paris Agreement, which seeks to limit temperature increases to 1.5 degrees Celsius over pre-industrial levels.