Spirit Airlines Inc. said it would entertain JetBlue Airways Corp.’s unsolicited $3.6 billion bid to buy the airline and start talks with its new suitor.

Spirit had already struck a $2.9 billion merger agreement with Frontier Group Holdings Inc., another budget airline, when JetBlue swooped in with a competing offer. Spirit said its board has determined that JetBlue’s offer “could reasonably be likely to lead to a ‘superior proposal’”—something that allows it to enter into talks with JetBlue under the terms of its merger agreement.