Aug 31 (Reuters) - A union representing Southwest Airlines Co (LUV.N) pilots has filed a lawsuit challenging forced time off and other changes to working conditions imposed by the airline during the COVID-19 pandemic. The Southwest Airlines Pilots Association filed a complaint in federal court in Dallas on Monday claiming that the carrier implemented an "emergency time off" program, altered schedules, and scaled back prescription drug and retirement benefits without bargaining, in violation of federal labor law. It claims Southwest should have collectively bargained with the union instead of giving itself "force majeure" rights when air travel plummeted during the pandemic. The lawsuit marks an escalation in mounting tensions between the airline and its staff. Its pilots union has threatened to picket over the winter holidays to protest against a host of issues including a gruelling work schedule, a lack of food and accommodation and COVID-19 protocols. The protest prompted the company last week to trim flight schedules for this fall in a bid to better align its operations with staffing. read more In the lawsuit, the union said the airline is bound by the terms of the collective bargaining agreement that lapsed in August last year, but remains in effect until a new agreement is reached and does not contain a "force majeure" clause. It asked the court for an injunction, requiring Southwest to stick to the provisions of the lapsed agreement, and negotiate the terms for an "emergency extended time off" program, and COVID 19-related work conditions. In an email sent to its members on Tuesday, the union said the lawsuit was the "only recourse" to compel the company to meet its duty to collectively bargain. Russell McCrady, Southwest vice president of labor relations, in a statement said that the airline disagrees that any COVID-related changes adopted in recent months required negotiation. "As always, Southwest remains committed to pilots’ health and welfare and to working with SWAPA, and our other union partners, as we continue navigating the challenges presented by the ongoing pandemic," he said.