• Frontier is offering a $250 million reverse breakup fee to Spirit.
• JetBlue Airways is trying to acquire Spirit and has launched a hostile takeover attempt.
• JetBlue’s offer includes a $200 million reverse breakup fee.

Frontier Airlines' parent company on Thursday said it would pay a $250 million reverse breakup fee to Spirit Airlines if regulators don't approve the planned combination of the two discount carriers for antitrust reasons, an effort aimed at convincing investors to approve the deal next week as rival JetBlue Airways tries to buy Spirit outright.

"The combination of a higher reverse termination fee and a much greater likelihood to close in a Frontier merger provides substantially more regulatory protection for Spirit stockholders than the transaction proposed by JetBlue," Mac Gardner, Spirit's chairman said in a news release.

New York-based JetBlue offered $33 a share, or $3.6 billion cash for Spirit, in April, above the $2.9 billion cash-and-stock deal that Spirit and Frontier announced in February.

Spirit's board rejected JetBlue's advances, and JetBlue last month made a tender offer of $30 a share and has urged Spirit shareholders to vote against the deal.

Spirit said a deal with JetBlue wouldn't likely be approved by regulators. JetBlue's offer includes a $200 million reverse breakup fee if regulators don't approve the acquisition.

JetBlue, for its part, said in a statement Thursday that the Spirit board "only went back to Frontier under pressure, when it became increasingly clear their shareholders would decisively reject the Spirit Board's flawed process and Frontier's inferior transaction." 

"The addition of a reverse termination fee in the face of a likely defeat is simply an acknowledgement that the regulatory profiles and timelines of both deals are indeed similar," the New York airline said.

Proxy advisory firm Institutional Shareholder Services had advised Spirit shareholders on Tuesday to vote against the Frontier deal, raising concerns about the lack of a reverse termination fee.

Spirit CEO Ted Christie told CNBC's "Squawk Box" on Friday that he is "hopeful" the reverse termination fee would get ISS to change its recommendation. ISS declined to comment.

Another proxy advisory firm, Glass Lewis, recommended early Friday that Spirit shareholders back the Frontier offer, noting that the "last-minute inclusion" of the reverse break-up fee should help ease concerns that the regulators could block the deal.

Spirit's shareholder meeting is set for June 10.