Luke Macgregor/Reuters

Few airline routes are as lucrative as the one between the financial powerhouses of New York and London. On Tuesday, Delta Air Lines signaled that it was going after that business-heavy market, agreeing to buy a 49 percent stake in Virgin Atlantic from Singapore Airlines for $360 million.

The deal will provide Delta with more access to Heathrow Airport, one of the world’s busiest hubs, where takeoff and landing rights are limited because of high demand and tight capacity. New York, where all major airlines are battling to attract high-paying passengers, is the top international destination from Heathrow.

Singapore bought its stake in 2000 for £600.3 million ($966 million), but it has been dissatisfied with the returns, analysts said. While Delta had considered buying Singapore’s stake two years ago, the carriers could not agree on a price.

On Tuesday, Delta and Virgin Atlantic said they would apply for antitrust immunity from American and European competition authorities to coordinate fares and flight schedules, as well as offering seats on each other’s planes. Virgin Group, headed by the British billionaire Richard Branson, has said it does not plan to sell its 51 percent controlling majority in Virgin Atlantic.

Delta, which has global ambitions, has a strong partnership with Air France-KLM that serves many European destinations, but it is not a strong contender in the London market. Delta has nine daily flights to Heathrow from New York, Boston and Atlanta. But it has no direct flights from other top markets like San Francisco, Chicago, Washington, Miami or Los Angeles, requiring passengers to connect through its other hubs.

Heathrow is operating at full capacity, and the British government has rejected expansion plans to build a third runway. As a result, takeoff and landing rights, known as slots, are limited, making them prized commodities for the airlines. In a sign of how valuable those slots are, Continental Airlines paid $209 million in early 2008 for four pair of takeoff and landing slots.

Delta has just 0.3 percent of the Heathrow slots, according to the Airport Coordination Limited, which is responsible for slot allocations at airports in Britain.

British Airways dominates Heathrow, with 53 percent of the slots, followed by Lufthansa of Germany, with 5.6 percent, and Virgin with 3.3 percent. American and United each have 2.3 percent.

British Airways’ hold on the airport actually increased in the last year after it completed the acquisition of British Midland International from Lufthansa. The acquisition was challenged by Virgin Atlantic, which claimed it would distort competition and simply reinforce the dominance of British Airways. The deal, however, was cleared by the European Commission in March under certain conditions, including that 14 of the 56 daily slot pairs British Airways received from British Midland be released to other carriers.

Still, Delta’s move is a challenge to American Airlines and British Airways, which are partners in the Oneworld global airline alliance. The two carriers dominate the New York to London market with 15 daily flights and a shuttlelike schedule of departures every 20 or 30 minutes in the peak evening hours. British Airways and American received antitrust immunity two years ago allowing them to coordinate their schedules and fares.

Virgin was founded by Mr. Branson in 1984 with flights to New York. From the start, it embraced an image of entertaining travel and cheaper fares. It now has 38 airplanes in its fleet and flies to more than two dozen destinations. But the airline, which is not aligned with any of the three global groups — Star Alliance, Sky Team and Oneworld — has struggled in recent years because of high fuel prices.

The Virgin Group owns 25 percent of Virgin America, a low-cost domestic carrier that is independent of its international namesake. American law forbids foreigners from owning more than 25 percent of a domestic airline. The European Union has a similar requirement barring non-European carriers from holding a majority stake in a European Union airline.

Air France-KLM is also considering buying part of Mr. Branson’s stake in Virgin Atlantic, according to reports in the British media. Such a deal, if it happened, would further strengthen Delta, which is a partner with Air France within the Sky Team alliance.

With the deal, Delta is wading into an old and often feisty rivalry between Virgin and British Airways.

Willie Walsh, who runs the parent company of British Airways, the International Airlines Group, said recently that Delta was really more interested in Virgin’s slots at Heathrow than in preserving the airline or its brand.

“I can’t see Delta wanting to operate the Virgin brand because if they do, what does that say about the Delta brand?” Mr. Walsh told Britain’s Telegraph newspaper. “Delta believes they are the No. 1 airline in the world, so what they would want to do is acquire the slots at Heathrow to enable them to have a strong presence at Heathrow.”

The comments drew a quick response from Mr. Branson, doubled with a characteristic challenge.

“Rumors have been spread in the press that I am planning to give up control of Virgin Atlantic and, according to Willie Walsh — who runs BA — that our brand will soon disappear,” Mr. Branson said on Monday in a statement titled “Sorry BA — we’re not going anywhere.”

The statement also said, “This is wishful thinking and totally misguided. Will BA never learn?”

Mr. Branson then offered to give employees of British Airways £1 million ($1.6 million) if Virgin Atlantic disappeared within the next five years. If it was still in business then, he challenged Mr. Walsh to pay the same amount to Virgin’s employees.

“Let’s see how much they believe this,” he said. “Let them put their money where their mouth is.”