shares jumped about 20% on Monday after the budget carrier and said after the market closed Friday that they would a blocking the airlines' planned merger on antitrust grounds.

"Our merger agreement with Spirit remains in effect and we still have obligations under the agreement. ... This is a standard procedure, required under the merger agreement," JetBlue General Counsel Brandon Nelson said in a note to staff Friday.

JetBlue agreed to buy Spirit for $3.8 billion in a that would have created the country's fifth-largest airline. U.S. District Court Judge William Young last week , citing reduced competition.

"JetBlue plans to convert Spirit's planes to the JetBlue layout and charge JetBlue's higher average fares to its customers," Young wrote in his Jan. 16 decision. "The elimination of Spirit would harm cost-conscious travelers who rely on Spirit's low fares."

Spirit shares are down more than 45% since that ruling. The stock had dropped more than 60% after the decision but has rebounded slightly following the appeal and after Spirit raised for the fourth quarter of 2023. The carrier also said it is looking to refinance its debt.

JetBlue shares rose about 1% on Monday and have climbed more than 3% since its merger with Sprit was blocked.

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