WASHINGTON, June 20 Reuters JetBlue Airways said Monday it had sweetened its takeover offer for Spirit Airlines to 33.50 per share in a bid to convince the ultralow cost carrier to accept its offer over rival Frontier Airlines39; proposal.

The move is the latest salvo in a tussle to create the fifthlargest U.S. airline, helping the buyer compete with larger legacy players at a time when the industry faces labor and aircraft shortages.

Spirits board had previously rejected the JetBlue offer, arguing that U.S. antitrust regulators would not approve a tie up with JetBlue and noting that JetBlue refused to abandon its alliance with American Airlines.

But Spirit said last week it was in talks with JetBlue over its offer and expected to decide on the proposal by June 30.

JetBlue39;s latest bid represents a 68 premium to Frontier39;s cash and stock offer, whose value stood at 19.99 per share as of Friday. Spirit shares closed up 2 to 21.28 on Friday.

The new proposal is 2 a share higher than its prior offer and includes what JetBlue termed a stronger divestiture commitment to complete the Spirit deal but does not include abandoning JetBlue39;s Northeast Alliance with American Airlines.

Spirit said Monday its board would work with financial and legal advisors to evaluate JetBlue39;s revised proposal and pursue the course of action it determines to be in the best interests of Spirit and its stockholders.

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JetBlue said it made the new offer at the request…