Southwest Airlines Co. pulled the Boeing Co. 737 Max from its schedule for an additional month, citing uncertainty over when regulators will allow the jet to resume service.

Southwest, the narrow-body aircraft’s largest operator, will cancel about 100 daily flights as the Max is removed from schedules through Sept. 2, the Dallas-based carrier said in a statement Thursday. The airline previously had the Max returning to service Aug. 6.

The delay comes as carriers wait for Boeing to complete fixes to Max software and for regulators to decide when the plane can resume flights. It was grounded worldwide on March 13, following two fatal crashes within five months.

American Airlines Group Inc. recently took the Max out of its schedule through Sept. 3, the day after Labor Day. The two carriers’ moves ground the plane at least through the traditional summer travel season. United Continental Holdings Inc., the other U.S. carrier operating the plane, plans to resume flights Aug. 4.

The Federal Aviation Administration has said there’s no time frame for signing off on Boeing’s proposed software fix and revised pilot training for the aircraft. An agency official said Wednesday that he expects the plane to be back in service this year.