JetBlue and Spirit Airlines announce a plan to merge, creating fifth-largest U.S. carrier.

JetBlue Airways said on Thursday that it had reached a deal to buy Spirit Airlines, a combination that could reshape the airline industry by putting pressure on the nation’s four dominant airlines.

The deal, which values Spirit at $3.8 billion, would create the nation’s fifth-largest airline, with a combined share of more than 10 percent of the market, behind United Airlines, which has a nearly 14 percent share.

The agreement is a victory for JetBlue, which successfully spoiled a rival effort to buy Spirit by Frontier Airlines. Frontier and Spirit had announced merger plans in February, but called that deal off on Wednesday, after struggling to convince Spirit’s shareholders to back its offer, which fell short of JetBlue’s by about $1 billion.

“Spirit and JetBlue will continue to advance our shared goal of disrupting the industry to bring down fares from the big four airlines,” Robin Hayes, JetBlue’s chief executive, said in a statement.

Spirit and JetBlue said they expected to seek approval for the deal from Spirit’s shareholders this fall and from regulators by the end of 2023 or early 2024. The airlines said they expect to close the transaction no later than the first half of 2024, with plans to begin operating as a single carrier by the first half of 2025.

But while the airlines have agreed to combine, closing the deal is far from certain. The Biden administration has taken a tough stance on antitrust, challenging corporate mergers that may reduce competition. Regulators have already sued JetBlue and American Airlines over a partnership at airports in Boston and New York.

Under the merger agreement, JetBlue would acquire Spirit for at least $33.50 per share in cash, significantly more than Spirit’s closing price of $24.30 on Wednesday. Spirit’s shares rose 4 percent in premarket trading, but remained below the price offered by JetBlue, reflecting skepticism about the deal.