U.K. Court Blocks Heathrow Airport Expansion on Environmental Grounds

LONDON — Environmental opponents of a plan to build a third runway at Heathrow Airport in London won a potentially decisive victory on Thursday after the Court of Appeal ruled that the project was unlawful because it violated Britain’s commitments to climate change.

The decision was a major victory for environmental groups and was one of the first legal rulings to invoke climate-related issues as grounds to strike down a major infrastructure project. The court said the runway could put Britain at odds with the commitments it made under the 2016 Paris climate accord to reduce carbon emissions.

On Thursday, Heathrow said it planned to appeal the court’s decision, and its chief executive, John Holland-Kaye, said, “Let’s be clear, no Heathrow expansion, no global Britain.”

Prime Minister Boris Johnson’s government could appeal the decision on the grounds that the project is important to the country’s post-Brexit economic future. But that seemed in doubt.

Mr. Johnson, whose parliamentary constituency encompasses towns and villages that are near the airport, which is about 14 miles west of Central London, has stridently opposed the expansion plan in the past.

In 2015, while he was the mayor of London, he promised constituents, “I will lie down with you in front of those bulldozers and stop the construction of that third runway.”

The decision leaves the long-debated runway in flux. At a minimum, the ruling will delay construction indefinitely, prolonging a battle over the runway that has raged for years. The $18 billion project would enable about 700 more planes a day to use Heathrow, but it would drive up the airport’s carbon dioxide emissions.

About 80 million passengers a year travel through Heathrow, one of the world’s busiest airports, and the traffic is already straining its capacity.

Proponents of the third runway argue that it is necessary to compete with other large European airports, including Charles de Gaulle in Paris, Schiphol in Amsterdam and Frankfurt in Germany.

The decision is the latest twist in a battle that has vexed both of Britain’s main political parties. The Labour Party government of Prime Minister Gordon Brown first gave the project a greenlight in 2009. His Conservative successor, David Cameron, reversed course, holding up the project. The Conservative government of Prime Minister Theresa May resurrected the plan in 2015.

The ruling is a big victory for Sadiq Khan, the mayor of London and a member of the opposition Labour Party, who campaigned against the runway.

“Today’s judgment is a major victory for all Londoners who are passionate about tackling the climate emergency and cleaning up our air,” Mr. Khan said on Twitter.