On Wednesday, shareholders of Spirit Airlines voted to accept JetBlue Airways’ $3.8 billion takeover, but the deal could still face challenges from federal antitrust regulators. JetBlue emerged as the winner in a bidding war with Frontier for Spirit, the largest US budget airline.

Spirit announced the results after a brief meeting online. Spirit said only that the JetBlue deal had majority support; it promised accurate calculations within four business days. After forcing Spirit to abandon its proposed merger with Frontier Airlines in favour of JetBlue’s more prosperous all-cash offer, Wall Street widely expected shareholders to approve the sale.

This is an essential step in completing a merger that will create the clearest national low-price challenger to any major US airline,” Spirit CEO Ted Christie said after the vote. JetBlue said in a statement, “This vote is an important milestone in our plan to partner with Spirit to create a high-quality, low-fare national challenger to the Big Four,” referring to American, United, Delta Air Lines and Southwest Airlines. JetBlue Airways vowed to work hard to pass the regulatory process.

JetBlue hopes to rebrand Spirit Aircraft and bring its pilots and other employees into the JetBlue workforce. The deal would make New York-based JetBlue the fifth-largest US airline with more than 450 planes and about 7,000 pilots, and it hopes to help it win customers from the major airlines.

However, it would also exclude Spirit, the largest US budget carrier, potentially a mismatch with regulators, who appear to oppose further consolidation in the airline industry after a round of mergers from 2005 to 2016.

The Justice Department is currently working to end the partnership of JetBlue and American Airlines in New York and Boston, which the airline calls the Northeast Alliance, or NEA. Lawyers for the department say the alliance is anti-competitive and will drive up consumer prices. Hearings that began last month in Boston federal court resumed on Monday.

Florian Adler, an antitrust expert and economics professor at Yale University, said the result of the NEA test could have a huge impact on whether the Justice Department allows JetBlue to buy Spirit or uses it to block the sale.

If (JetBlue and American) prevail and a judge rules that NEA did not cause substantial harm to consumers, the acquisition of Spirit will almost certainly face an antitrust challenge, Ederer said. JetBlue believes the alliance with America should be allowed because it is not a merger. However, acquiring Spirit would combine the two airlines. JetBlue CEO Robin Hayes said he was confident of getting regulatory approval to buy Spirit. The airline hopes to complete the sale in the first half of 2024.

Spirit and Frontier announced plans to merge in February. Both are so-called ultra-low-cost airlines that charge lower fares than other airlines but charge more to make up some of the difference. JetBlue tried to buy Virgin America in 2016 but lost a bidding war with Alaska Airlines and outbid Frontier in April with a stock and cash offer. JetBlue overcame objections from Spirit’s board and management to oust Frontier. JetBlue’s proposal calls for Spirit shareholders to receive $33.50 per share in cash, including an upfront payment of $2.50 per share after Spirit shareholders approve the transaction and a monthly fee of 10 cents as regulators review the matter.

If regulators terminate the sale, JetBlue will pay Miramar, Florida-based Spirit $70 million and Spirit shareholders $400 million. It’s possible that Frontier could benefit from the Spirit sale even after the bidding war fell through. If Spirit disappears, Denver-based Frontier will become the largest budget airline in the US, serving the most price-sensitive travellers.