A federal judge on Tuesday blocked JetBlue Airways’ effort to merge with low-cost rival Spirit Airlines, handing the Biden administration a significant victory in its effort to preserve competition in a key industry that critics say has grown too concentrated.

In his 113-page decision, U.S. District Court Judge William G. Young wrote: “A post-merger, combined firm of JetBlue and Spirit would likely place stronger competitive pressure on the larger airlines in the country. At the same time, however, the consumers that rely on Spirit’s unique, low-price model would likely be harmed.”

JetBlue and Spirit had no immediate comment. The Justice Department also did not immediately respond to requests for comment.

When the Justice Department filed its challenge in March, it argued that a merger should be blocked because the loss of Spirit would reduce competition in a concentrated industry at the expense of cost-conscious travelers who depend on Spirit’s cheaper fares.

“This case is about harm – significant harm to more than millions of real people trying to get where they need to be,” Justice Department lawyers argued in their post-trial brief. “Defendants had both the opportunity and the responsibility to propose a deal that does not threaten harm – a deal that does not violate the law. Instead they chose to champion the deal before the Court, an illegal merger that they attempted to salvage through incomplete divestitures and fanciful predications about the future.”

JetBlue countered by saying the $3.8 billion deal between the nation’s sixth- and seventh-largest carriers would create a larger airline better equipped to take on American Airlines, Delta Air Lines, Southwest Airlines and United Airlines, which dominate the market. A merger of JetBlue and Spirit would create the nation’s fifth-largest carrier.

In post-trial briefs, JetBlue said the merger would benefit consumers.

“Through this transaction, JetBlue would become a viable, long-term, national challenger to the dominant legacies, spreading the JetBlue effect – along with billions in consumer savings across the country,” the carrier contended.

In his decision, Young wrote: “There are no ‘bad guys’ in this case. The two corporations are – as they are expected to – seeking to maximize shareholder value. The Department of Justice is – as the law requires – speaking for consumers who otherwise would have no voice. Their forthrightness, civility, and zealous advocacy have immeasurably assisted the Court in reaching out for justice.”

The Justice Department’s challenge to the merger was the first such heard by a judge. The department has challenged previous airline mergers, but ultimately allowed the deals to go forward after negotiating concessions with carriers.

The merger would have been the industry’s first in seven years, when Alaska Airlines merged with Virgin America. JetBlue also sought to acquire Virgin but was outbid. Another industry merger is also in the works: Days before closing arguments in the JetBlue case, Alaska Airlines announced plans to merge with Hawaiian Airlines in a deal valued at $1.9 billion. That deal is also expected to face close scrutiny from regulators.

Scott Keyes, founder of travel website Going and a longtime industry watcher, said the Biden administration’s effort to block JetBlue’s merger with Spirit underscores the critical role low-cost carriers play in the market. When Spirit enters a market, he said, fares go down. Even so, he said waves of consolidation have made it difficult for smaller players to compete against larger carriers, leading airlines like JetBlue to seek mergers for growth.

More than 22 witnesses, including chief executives of Spirit and JetBlue, testified, and 900 exhibits were shown during the four-week trial, which began at the end of October in U.S. District Court in Boston.

Young’s decision comes as JetBlue and Spirit are facing financial headwinds. On the same day the trial began, JetBlue reported a $153 million quarterly loss, while Spirit recently launched cost-saving measures.

The Justice Department won an earlier challenge in a case that involved JetBlue and an alliance it formed with American Airlines that allowed the two carriers to coordinate schedules and share revenue on certain routes in the Northeast. The airlines unsuccessfully argued that the arrangement, known at the Northeast Alliance, enabled them to compete with United and Delta, major players in the New York and Boston markets. The Justice Department, however, called the arrangement a de facto merger.