By Jonathan Stempel

(Reuters) -Southwest Airlines must face a lawsuit accusing the carrier of illegally intimidating and disciplining pilots who participate in its more than 9,000-member pilots union, a federal appeals court ruled on Monday.

In a 3-0 decision, the 5th U.S. Circuit Court of Appeals said the Southwest Airlines Pilots Association sufficiently pleaded that the Dallas-based carrier had "anti-union animus" to allow the dispute to proceed in federal court.

A lower court judge had ruled that the dispute was minor and therefore belonged in arbitration.

The case stemmed from Southwest's decision to strip Timothy Roebling of his responsibilities and pay as a "check pilot," a special group of about 300 pilots who work closely with management and train other pilots.

Southwest ostensibly cited Roebling's use of a vulgarity to justify the discipline, but the union said it resulted from his decision to join the union's check pilot committee.

Writing for the New Orleans-based appeals court panel, Chief Judge Jennifer Walker Elrod agreed with the lower court judge that the dispute was a minor matter under the federal Railway Labor Act.

But she said accusations that Southwest made check pilots fearful of retaliation for associating with the union, and that Roebling's boss threatened to strip him of his qualifications, made this case different.

These kinds of allegations "sufficiently support the union's claim that Southwest intended to 'weaken' or 'destroy' the operational capacity of the union," she wrote.

Southwest said in a statement it disagreed with the court's decision and vehemently denied the union's allegations. "We are evaluating our options," it added.

Lawyers for the union did not immediately respond to requests for comment.

The appeals court returned the case to U.S. District Judge Barbara Lynn in Dallas federal court, who had dismissed it in September 2023.

The case is Southwest Airlines Pilots Association v Southwest Airlines Co, 5th U.S. Circuit Court of Appeals, No. 23-11065.

(Reporting by Jonathan Stempel in New York; Editing by Richard Chang)