A Frontier Airlines jetliner taxis to a runway for take off from Denver International Airport Thursday, May 26, 2022, in Denver. Experts are expecting record crowds at airports and on the nation's byways during the long Memorial Day weekend, which marks the start of the summer travel season, in spite of high fuel costs. (AP Photo/David Zalubowski)

Frontier Airlines sweetened its offer for Spirit Airlines on Thursday by adding a $250 million termination fee if Frontier fails to complete the acquisition of the rival discount carrier.

The new offer was endorsed by directors of both airlines.

Spirit CEO Ted Christie said shareholders have indicated support for the strategic reason to combine with Denver-based Frontier, but expressed “a desire for additional stockholder protections. After discussing this feedback with the Frontier board and management team, we have agreed to amend the merger agreement.”

JetBlue Airways, which is seeking to derail the deal and buy Spirit itself, had argued that the lack of a break-up fee was a major weakness in Frontier’s offer. New York-based JetBlue included a $200 million fee in its offer.

Shareholders of Florida-based Spirit are scheduled to vote June 10 on Frontier’s stock and cash offer, which has the unanimous support of the Spirit board. The airlines valued the deal at $2.9 billion when they announced it, but it has lost about a quarter of its value of a drop in Frontier’s stock price.

JetBlue originally offered $3.6 billion in cash, then launched a $3.2 billion tender offer and asked Spirit shareholders to reject the Frontier bid.

This week, a firm that advises investors on proxy voting said Spirit shareholders should oppose the Frontier bid to buy Spirit because the JetBlue offer is financially better.

Spirit argues that antitrust regulators would block JetBlue from buying Spirit, in part because of a partnership JetBlue has with American Airlines in the Northeast. The Justice Department sued JetBlue and American to block that deal, and a trial is scheduled for this fall. Also, Spirit shareholders will keep 48.5% of the combined company under the Frontier deal.