A federal judge established a partnership between American Airlines and JetBlue Airways at New York and Boston airports, writing Friday in a ruling that the alliance would hurt competition and raise fares.

The decision is a big win for the Justice Department, which under President Biden has sought to more aggressively enforce antitrust laws, especially in industries such as air travel and technology, where a few companies are so dominant that it can be difficult, if not impossible, for small businesses. to challenge them. The judge ruled that the airline partnership known as the Northeast Alliance must end.

Under the agreement, effective in 2021, each airline sells seats offered by the other on certain routes. The airlines also share revenue from certain flights and gate access at airports. The alliance covers three major airports serving New York and Boston’s Logan International Airport.

The Justice Department said the cooperation reduces competition and will cost travelers hundreds of millions of dollars a year if it stays put. The airlines argued that the partnership provides consumers with more options to fly.

Endorsing the government, U.S. District Court Boston Judge Leo T. Sorokin wrote: “This makes the two airlines partners, each with a vested interest in the success of their joint and individual efforts, rather than vigorous, independent rivals regularly challenging each other in a competitive marketplace.” “.

JetBlue and American said in statements that they are considering their legal options. JetBlue said it was “disappointed with the decision”, while American called the decision “clearly wrong”. Both described the partnership as a “huge win” for clients.

For the Justice Department, “this is a big win,” said Gene Kimmelman, a fellow at the Harvard Kennedy School and the Yale Center for Economic Policy. Tobin, as well as a former employee of the Department of Justice. “This was a very important case for the department to show that the agreement was built as a merger that concentrates power at key hub airports by coordinating flight schedules and airline capacity.”

A series of mergers over the course of about two decades has significantly reduced the number of major airlines in the United States. In 2013, for example, American merged with US Airways. Previously, United and Continental Airlines became one company, and Delta Air Lines merged with Northwest Airlines. This has left travelers with fewer choices, especially at many hub airports, which tend to be dominated by one or two airlines.

Part of the Justice Department’s concern was that other airlines could also enter into partnership agreements, further limiting choice for customers.

The decision dealt a blow to JetBlue, which has been looking to expand rapidly in recent years. In addition to the alliance with American, JetBlue made a deal to buy Spirit Airlines. The Ministry of Justice asks the judge to block this acquisition as well.

JetBlue is the sixth largest airline in the US, with a 5.5% domestic market share, according to federal data. The American is the largest, with 17.6 percent.

In a lawsuit against the Northeast Alliance, the Justice Department argued that JetBlue had a disruptive effect on the industry by forcing larger, more established airlines to cut prices. JetBlue’s deal with American effectively eliminated a major competitor from several important markets, the department said.

More than 75 percent of all JetBlue flights last year flew to or from the four airports covered by the agreement, according to flight schedules tracked by Cirium, an aviation data company.

“While the defendants claim that their cooperation on the principle of “the more the better” will benefit passengers, they have presented a minimum of objective credible evidence to support this claim,” Judge Sorokin wrote. “Whatever the benefits to American and JetBlue in becoming more powerful—in the Northeast as a whole, or in their overall rivalry with Delta—such benefits come from an open agreement not to compete with each other.”

Airline share prices tumbled about 1.5 percent on Friday, but after the court ruling later in the day, additional selling pressure on the secondary market appeared to be negligible.

American and JetBlue have recorded significant market value gains this year, but both companies still have a long way to go before they recover from the pandemic’s devastating impact on air travel: American has lost about half its market value since early 2020, while Since then, JetBlue shares have fallen more than 60 percent.

In a lawsuit aimed at preventing a takeover of JetBlue Spirit, antitrust regulators have argued that JetBlue’s pursuit of the Northeast Alliance indicates that the airline is increasingly operating as a larger and more established carrier. Today, Spirit is even more disruptive to other airlines than JetBlue, which has “less reason to continue to compete aggressively” with the country’s largest airlines, the department said. The case is expected to go to court this year if it is not settled sooner.