LONDON — Britain’s Court of Appeal issued a landmark ruling on Thursday that stymied plans to build a third runway at Heathrow Airport in London, declaring that the government illegally neglected its commitments to reduce carbon emissions and protect the planet from dangerously high temperatures.
The ruling, among the first in the world to measure a state’s infrastructure plans specifically against its promises under the Paris Agreement on climate change, threw the expansion of Heathrow into doubt and opened up a new frontier for legal challenges to major projects in Britain and around the world.
While the decision left the door open for Britain’s prime minister, Boris Johnson, to reformulate the plans and try again, it set back the runway expansion considerably, prolonging a battle that has raged for years. That posed a dilemma to a country — a “truly global Britain,” in Mr. Johnson’s words — that is in the midst of severing its strong ties to Europe and looking for new trading partners farther afield.
But analysts said the ruling also relieved Mr. Johnson of having to oversee a project that he once opposed — so strongly, he once said, that he would lie down in front of bulldozers to stop it. And it halted one part of the government’s aviation expansion plans at a moment when any additional flights jeopardize Britain’s own legally binding target of net-zero greenhouse gas emissions by 2050.
While the government said it would not appeal the ruling, the owners of Heathrow said on Thursday that they would.
Technically speaking, the ruling said only that the expansion plans — drawn up under Mr. Johnson’s predecessor, Theresa May — needed to take into account Britain’s commitments to help reduce global warming, not that the plans definitely clashed with those vows. That left room for the government, if it wished, to make modifications to its case for the new runway.
But with Mr. Johnson’s government already divided on the expansion plan, the ruling made it less likely that he would quickly press ahead, analysts said.
For the Paris accord — already under threat from President Trump, who pulled the United States out of the agreement last year — the ruling sent a strong signal about its potential to serve as a check on domestic policies related to energy and infrastructure. Rulings by British courts, while not binding elsewhere, can influence legal thinking in other jurisdictions.
“It’s not enough to take into account what you said you’d be doing under your own legislation,” said Jorge E. Viñuales, the Harold Samuel professor of law and environmental policy at the University of Cambridge, describing the message the ruling sent. “You suddenly have to take into account the declarations that states made under the Paris agreement, which is a completely new dimension that really bites.”
“It’s a very significant symbolic endorsement of the Paris agreement at a time when that’s really welcome,” he added, “because it’s navigating difficult waters.”
Buoyed by the victory, environmental campaigners in Britain were already thinking about other projects that could be modified or struck down on the same grounds: regional airport expansions, major railway projects. And legal experts said they expected the ruling to inspire cases in other countries, too.
“It opens up so many doors it’s almost hard to know which ones to choose,” said Tim Crosland, the director of Plan B, one of the groups that brought the challenge. “This is absolutely what the courts are for, to say, ‘Stop, this doesn’t make sense, governments are failing in their basic responsibilities to protect people.’”
The ruling is part of a string of recent successes for environmental campaigners. In December, the Supreme Court of the Netherlands ordered the government to cut the country’s greenhouse gas emissions by 25 percent from 1990 levels by the end of 2020.
“We’re now beginning to see the courts play a more active role, not just in implementing environmental law but in plugging gaps in some sense,” said Lavanya Rajamani, a professor of international environmental law at the University of Oxford. “They’re being used very strategically, and it’s beginning to bear fruit.”
The Heathrow runway case hung on a decade-old British planning law that requires the government to take into account its climate policy in making infrastructure plans.
The government interpreted that to mean that it needed only to consider domestic British legislation related to climate change, not international commitments like the Paris Agreement. But the courts disagreed, saying that the Paris Agreement fell squarely within the boundaries of government climate policy.
“It is clear, therefore, that it was the Government’s expressly stated policy that it was committed to adhering to the Paris Agreement to limit the rise in global temperature to well below 2 degrees Celsius and to pursue efforts to limit it to 1.5 degrees Celsius,” the ruling said. But, the decision said, the secretary of state for transport “did not take the Paris Agreement into account at all. On the contrary, as we understand it, he consciously chose — on advice — not to take it into account.”
Pressure for more runway space at British airports has bedeviled governments for decades. Heathrow, a major hub for global travel that handles 80 million passengers a year, argued that it needed the extra runway to avoid being overtaken by other large European airports, including Charles de Gaulle in Paris, Schiphol in Amsterdam and Frankfurt in Germany.
The $18 billion runway project would enable about 700 more planes a day to use Heathrow, but it would drive up the airport’s carbon dioxide emissions, not to speak of the extra noise.
Business groups bemoaned the court’s decision on Thursday, saying that companies were at risk of losing access to trading markets around the world.
“There has never been a more important time to demonstrate that Britain is open for business,” said Adam Marshall, director-general of the British Chambers of Commerce, alluding to Brexit. “The government must back Heathrow expansion unequivocally and take all necessary steps to finally move the project forward.”
But for Mr. Johnson, whose parliamentary constituency encompasses towns and villages that are near the airport, which is about 14 miles west of Central London, the decision may have come as something of a reprieve.
He has avoided taking strong public stands on the expansion plans in recent years; in 2018, when Parliament backed the plan in a vote, Mr. Johnson absented himself by traveling to Afghanistan in his role as foreign secretary.
Still, said Tony Travers, a professor of government at the London School of Economics, the ruling only deferred a decision on a matter that has vexed Britain’s main political parties. The Labour Party government of Prime Minister Gordon Brown first gave the project a green light in 2009. His Conservative successor, David Cameron, reversed course, holding up the project. The Conservative government of Mrs. May resurrected the plan in 2015.
And now Mr. Johnson’s Conservative Party, which publicly supports environmental protections even as its most pro-Brexit factions show some willingness to abandon them, now has to figure out its own approach.
“Giving up on having a major international hub airport would be a noble thing to do,” Mr. Travers said. “But it’s not, I suspect, going to be enough to convince the French or the Dutch not to continue with theirs, and the British government needs to think about all of this.”
Anna Joyce contributed reporting.