Southwest Airlines has filed a federal complaint alleging union leaders told mechanics to decline to work overtime, a move the carrier says has harmed operations and driven up costs.

Southwest said in the lawsuit filed Wednesday in Dallas that the Aircraft Mechanics Fraternal Association encouraged workers earlier this month to decline overtime opportunities.

The Dallas-based carrier says it relies on the overtime shifts to keep up with necessary maintenance work.

The company says there was a 75 percent drop in union workers who signed up for overtime shifts last weekend.

Southwest says the union action caused “irreparable injury,” in part by having to rely on outside workers.

A union statement cautions members against any organized attempt to avoid overtime.

This article was from The Associated Press and was legally licensed through the NewsCred publisher network.

Photo Credit: Southwest alleges that a labor action by mechanics is hurting repair operations. Pictured is an employee of the airlines cargo unit, which is not involved in the job action, on October 7, 2016. Southwest Airlines