Spirit Airlines and Frontier Airlines, two prominent budget carriers, announced plans to merge Monday, a combination that would create the fifth-largest U.S. airline by market share, putting pressure on the nation’s biggest carriers and raising concerns about further consolidation in an already-concentrated industry.

The airlines, which offer 1,000 daily flights serving destinations in the United States, the Caribbean and Latin America, said in a statement that customers would save $1 billion annually from the merger and that the airlines would not lay off employees because of the merger. They also said they expected to hire another 10,000 workers by 2026 to add to their combined 15,000 employees currently.

The deal could face pushback from the Biden administration, which has increasingly challenged such combinations in court. In the fall, the Justice Department sued to prevent a domestic alliance between American Airlines and JetBlue Airways, arguing that the agreement would drive up prices and reduce competition.

The U.S. airline industry has seen a tremendous amount of consolidation over the past two decades, with the nation’s four largest airlines controlling about 80% of the domestic market. Spirit and Frontier argue that the merger would allow them to better challenge those large carriers. But it would also create a giant budget airline that could smother smaller companies, including two recent budget entrants, Breeze and Avelo.

“We basically have a four-firm oligopoly in the national market,” said Diana Moss, president of the American Antitrust Institute, a left-leaning think tank and competition-law advocacy group. “Having this fringe of smaller carriers breathing down their necks is really the only thing left that keeps the Big Four on their toes.”

Barry Biffle, Frontier’s chief executive, said the airlines had talked with the administration about the merger and expected it would be well-received, arguing the deal would allow them to offer more cheap fares and better service.

“The administration reached out to us, as well as Spirit and other low-cost carriers, over the last year asking us how they could do more for competition,” he said in an interview. “And I think one of the big answers to that is this merger because we have to have the scale and ability to compete against the Big Four.”

The deal comes as the airline industry strives to move past the pandemic. Executives expect the recovery to accelerate in the spring and summer. While every carrier was devastated over the past two years, Spirit and Frontier have been able to build back more quickly thanks to an early rebound in domestic leisure travel, their core business. Corporate and international travel have been slower to recover.

The merger is expected to close in the second half of the year, subject to regulatory review and approval of Spirit shareholders. Under the deal, Frontier would buy Spirit for $2.9 billion in stock and cash. Little has been decided about how the new company would operate, including its management team, branding and the location of its headquarters.

Under the agreement, owners of Frontier’s equity would control 51.5% of the combined company, and Frontier would name seven of 12 board members. The board would be headed by William A. Franke, chairman of Frontier and managing partner of Indigo Partners, a private equity firm that invests in budget airlines.

Indigo has a controlling interest in Frontier’s parent company, Frontier Group Holdings, and held a controlling interest in Spirit from 2006 to 2013. Under Indigo’s leadership, Spirit went public in 2011, and Frontier went public last year. The private equity firm also has advised and invested in Tigerair in Singapore, Volaris in Mexico and Wizz Air in Europe. Biffle, Frontier’s chief executive, was a top Spirit executive from 2005 to 2013.

“Indigo has a long history with both Spirit and Frontier,” Franke said in the Monday conference call. “I think it’s safe to say no one knows them better than I do.”

A merger of the two airlines has long been the subject of speculation. Analysts have contended that the airlines complement one another. Frontier, headquartered in Colorado, is more heavily concentrated in Western states. Spirit, which is based in Florida, is more concentrated in the East. While the airlines sometimes serve the same cities, they only overlap in about 18% of their routes, according to Cirium, an aviation data provider.

Spirit brings international exposure, with nearly three times as many flights abroad as Frontier, according to Cirium. Together, the airlines also said, they would be able to serve destinations that one or both had previously abandoned, including Jackson, Mississippi; Birmingham, Alabama; and Dulles International Airport near Washington. The merger also could enable the new airline to start flights to small cities, including Eugene, Oregon; Ithaca, New York; and Worcester, Massachusetts, they said.

The airlines argued that the merger will benefit consumers. In November, the average price of a domestic ticket sold by Spirit was $109, before taxes and fees, compared with $73 for Frontier, according to Cirium. By joining forces, the airlines assert they would be able to offer more flights on existing routes, giving customers more choices and allowing the new company to better respond to disruptions.

“I think it’s a slam dunk, not a reduction of competition,” said Robert Mann, an industry analyst and consultant. “It essentially reinforces the price discipline that DOJ relies on when they allow other things which arguably aren’t so good.”

But the combination would also consolidate the airlines’ hold over some airports. Together, Spirit and Frontier would hold a 26% share of the market in Orlando, Florida, more than any other airline, according to Cirium data for 2021. In Las Vegas, the combined carrier would have a 24% share, second only to Southwest Airlines.

As a result, the deal could pressure other carriers, such as JetBlue, Alaska Airlines, Hawaiian Airlines and Allegiant Air, to join forces through partnerships or mergers.

In addition to regulatory approval, Spirit and Frontier will have to renegotiate contracts with their respective unions, which were notified of the deal Monday. Pilots at both airlines are represented by the Air Line Pilots Association, while the flight attendants for both are represented by the Association of Flight Attendants.

“Our first priority is to determine whether this merger will improve conditions for flight attendants just like the benefits the companies have described for shareholders and consumers,” the flight attendants union said in a statement. “Our support of the merger will depend on this.”

Spirit and Frontier have a combined fleet of more than 280 planes, with plans to grow to nearly 500 by 2026.

Spirit’s stock was up about 15% in midday trading Monday, though it remained below Frontier’s bid of $25.83 per share, implying some skepticism that the transaction would be completed.

Other airline stocks were also up on the news, which is not usually how shares of competitors react to the potential entry of a “disruptive” new challenger. It was another sign that Frontier and Spirit could face a challenge in convincing regulators that their merger would actually lead to stiffer competition and lower prices.

The combined airline would have annual revenue of approximately $5.3 billion based on 2021 results, the announcement said.

This article originally appeared in The New York Times.