A sudden bid from JetBlue for Spirit Airlines has thrown a Frontier acquisition deal off-kilter, writes Erik Bojansky for South Florida Business Journal, as reported in the Philadelphia Business Journal.

New York-based low-cost airline JetBlue Airways presented an unsolicited $3.6 billion proposal Tuesday.

Spirit is the fourth-largest carrier at Philadelphia International Airport. Frontier is the second-largest. Combined, the two would create the fifth largest airline in the U.S.

JetBlue’s offer rivals Frontier Airlines, which agreed to pay nearly $3 billion for Spirit in February. 

The Spirit board of directors ” will work with its financial and legal advisers to evaluate JetBlue’s proposal and pursue the course of action it determines to be in the best interests of Spirit and its stockholders,” Spirit representatives stated. “That evaluation will be in accordance with the terms of Spirit’s merger agreement with Frontier.”

JetBlue, which has a Northeast Alliance with American Airlines, said its offer for Spirit would be a “game-changer” creating a major competitor to the four dominant U.S. passenger carriers.

A Frontier spokesman countered that a combined Spirit/Frontier would deliver $1 billion in annual savings for consumers and offer more ultra-low fares to more places nationwide.

Read more at The Philadelphia Business Journal about a new bid for Spirit Airlines.