LONDON — After decades of equivocation, the British government said on Tuesday that it favored building a third runway at London’s main airport, Heathrow, although it plans to delay a vote in Parliament, probably for a year, to allow critics time to muster opposition.
The announcement was intended to put an end to years of political paralysis over aviation planning in the southeast of England, a situation that has led to Heathrow operating at 98 percent of capacity and Britain losing ground to Continental European air hubs.
Airport expansion has been one of the most crucial domestic political tests facing Theresa May, the new prime minister who in the past had opposed it. The proposal she offered on Tuesday is a departure from her predecessor, David Cameron, who had overturned a Heathrow expansion plan offered by a previous Labour administration.
“A new runway at Heathrow will improve connectivity in the U.K. itself and crucially boost our connections with the rest of the world, supporting exports, trade and job opportunities,” the transportation secretary, Chris Grayling, said in a statement before he was to address members of Parliament. “This isn’t just a great deal for business, it’s a great deal for passengers who will also benefit from access to more airlines, destinations and flights.”
The debate over expanding Heathrow has raged since the 1970s, when a government document described the airport’s capacity as “restricted.” Its location, about 17 miles west of central London, puts flight paths over many residential districts and over the constituencies of influential lawmakers, including Boris Johnson, the former mayor of London who is now Britain’s foreign secretary.
Mr. Johnson has said he would lie down in front of the bulldozers were the airport expansion to go ahead, and another Conservative member of Parliament, Zac Goldsmith, had promised to resign his parliamentary seat in protest, which would prompt a special election that could reduce the party’s already slim majority in the House of Commons. On Twitter, Mr. Goldsmith called the decision “catastrophic.”
On Tuesday, Ms. May gave ministers like Mr. Johnson the freedom to oppose the government’s decision — at least for a time — and pushed back a parliamentary vote on the project, probably for a year. That move, along with likely legal challenges, is expected to delay any prospect of construction into the next decade.
Political sensitivities had paralyzed decision-making by Mr. Cameron, who resigned as prime minister after Britain’s surprise decision, in a June 23 referendum, to leave the European Union. With Ms. May having set a March deadline for starting the legal process for leaving the 28-nation bloc, the airport decision is seen as a symbol of her government’s commitment to pursue an outward-looking, trade-friendly strategy.
Three main options had been under consideration before the announcement on Tuesday: building a third runway at Heathrow; extending one of its existing runways; or constructing a second runway at Gatwick, the second-busiest airport serving the capital, about 26 miles south of central London.
Sadiq Khan, the opposition Labour politician who succeeded Mr. Johnson as mayor of London, has backed the expansion of Gatwick.
Gatwick has half the traffic of Heathrow and one runway; Heathrow’s main European rivals as hub airports already have more runways than Heathrow: Schiphol in Amsterdam has six; Frankfurt and Charles de Gaulle in Paris each have four.
Heathrow is not a cheap option, financially as well as politically. It comes with a price tag of 18 billion pounds (around $22 billion) and will involve the demolition of hundreds of homes.
But Heathrow has been considered the most viable option for expansion, because of the surrounding infrastructure and because it is already a global aviation hub and the biggest handler of cargo.
It emerged as the top choice from a report by Howard Davies, chairman of the Airports Commission, who concluded that a new northwest runway for Heathrow was a much better alternative for expansion than a new runway at Gatwick, both for passengers and freight.
The Heathrow choice would add £147 billion to economic growth and create 70,000 jobs by 2050, and would provide better connections to the world for Britons, including those living outside London, the commission said.
A new runway would allow Heathrow to double its passenger capacity by 2050 and increase annual takeoffs and landings to 740,000 from about 480,000, reducing costs for passengers.
“The country has waited nearly 50 years for this decision,” said Paul Drechsler, president of CBI, one of Britain’s largest business groups.
He called the proposal a relief and added, “Our aviation capacity is set to run out as early as 2025, so it’s crucial we get spades in the ground as soon as possible.”
The expansion, he said, “will provide not only a welcome economic stimulus but will show the world that we are well and truly open for business as we negotiate our exit from the E.U.”
Dan Lewis, senior adviser on infrastructure policy at the Institute of Directors, another large business group, likewise applauded the decision, but he called for interim solutions, including increasing capacity on the train line from London to Gatwick and lifting the annual cap on flights from Stansted, another airport serving London. He noted that the expected legal challenges could delay Heathrow’s expansion, and he urged government ministers to keep open the option of a second runway at Gatwick.
Olivier Jankovec, director general of ACI Europe, a trade association of European airlines, said the announcement “brings us closer to the end of one of the longest, most publicly consulted infrastructure planning processes anywhere in the world.”
He added: “An island economy lives or dies by its air connectivity. If the U.K. government is serious about its focus on economic growth and preserving the country’s global positioning, it needs to truly embed air connectivity and sustainable airport development in its economic strategy.”
Environmental advocates have said that a Heathrow expansion would hinder efforts to reduce carbon emissions. They argue that passengers will be saddled with charges for more efficient planes, biofuels and carbon taxes in the years ahead.