US airline JetBlue has made a $3,600 million bid to acquire low-cost rival Spirit, in a move seeking to thwart the sale of that company to Frontier Group, which was agreed in February but has not yet closed.

Spirit announced in a statement Tuesday that it had received an “unsolicited offer” from JetBlue to acquire all of its shares at $33 per share.

This offer would in principle override the pool of shares and cash that Spirit shareholders would have received under the agreement reached with Frontier last February that is still awaiting final approval.

Spirit, in its memo, indicated that the company’s board of directors will work with its financial and legal advisors to evaluate JetBlue’s proposal and choose the option that is in the best interest of the company and its shareholders.

The low-cost airline’s statement confirmed earlier information from the New York Times, published shortly before the close of the trading session that caused Spirit shares to rise 22%, to $26.91.

Meanwhile, shares of JetBlue – which has not commented publicly – closed down more than 7% in response to news of its offering.

Florida-based Spirit and Frontier in February announced an agreement to create the fifth largest airline in the United States, which, according to the companies, will improve competition in the sector by offering a real competitor to the four American giants (American Airlines, Delta, United and Southwest).

JetBlue, which focuses its operations in the east of the country, is the sixth largest US airline.