The bidding war over Spirit Airlines is ramping up again, with JetBlue boosting its offer for the discount carrier just days after rival Frontier increased its own bid for Spirit.

Spirit shareholders are also scheduled to vote Friday on Frontier’s offer. The Frontier bid had been valued at $2.9 billion at the time of the cash-and-stock offer but that value has since dropped.

Spirit’s board has supported the Frontier deal because of uncertainty about JetBlue’s ability to win approval of its $3.6 billion offer from antitrust regulators.

JetBlue said Monday that it will now provide a $350 million reverse break-up payable to Spirit if a deal between the two isn’t completed for antitrust reasons. This is $150 million more than JetBlue previously offered to pay.

JetBlue said it would prepay $1.50 per share in cash, about $164 million, of the reverse break-up fee, in the form of a cash dividend to Spirit stockholders promptly following the Spirit stockholder vote approving the combination between Spirit and JetBlue.

Spirit stockholders would ultimately receive $30 per share in cash at the closing of the transaction and the prepayment of $1.50 per share of the reverse break-up fee.

Spirit did not immediately respond to an email seeking comment.