JetBlue Airways Corp. improved its offer for Spirit Airlines Inc., boosting a breakup provision to $350 million and adding an upfront cash payment just days before shareholders will vote on a pending buyout agreement with Frontier Group Holdings Inc.

The revised offer increases JetBlue’s reverse breakup fee by $150 million and provides for about $164 million payable as a cash dividend “promptly following” a vote approving a combination of the carriers, the airline said in a statement Monday. The update comes after Frontier sweetened its own agreement by adding a key $250 million fee payable to Spirit if their accord breaks up on antitrust grounds.

JetBlue is aiming to build more support among Spirit shareholders for its higher, all-cash offer ahead of a June 10 vote. It needs them to vote against Frontier’s stock-and-cash deal, initially valued at $2.9 billion, to preserve its best chance for a quick infusion of growth that will help it compete against larger US carriers. Spirit rejected JetBlue’s initial $3.6 billion offer, prompting a subsequent $3.3 billion hostile tender bid.

Spirit shareholders are faced with conflicting recommendations from prominent shareholder advisory firms. Institutional Shareholder Services Inc. found JetBlue’s all-cash offer superior from a financial standpoint and that both bids have inherent risks when it comes to federal antitrust reviews. A rejection by Spirit investors would signal their board to restart talks with JetBlue, it said.

Proxy advisory firm Glass Lewis & Co. subsequently recommended Frontier’s proposal, calling it “the best available and most actionable” alternative.

Shares of Spirit jumped 7.5% in the premarket to $22.30. JetBlue was down 0.2% to $10.45.