A Boeing employees works outside of the cockpit of a Boeing 737 MAX 8 airplane in the company's factory, on March 27, 2019 in Renton, Washington.

Boeing on Tuesday recommended pilots of the 737 Max undergo simulator training before the beleaguered jets can return to service, a step that could lead to further delays for airline customers.

"This recommendation takes into account our unstinting commitment to the safe return of service as well as changes to the airplane and test results," Boeing said in a statement. "Final determination will be established by the regulators."

The decision is a sharp shift for Boeing, which had previously expected pilot training to be computer-based. Easily transitioning pilots from older models of the 737 to the Max when it debuted in 2017 was a key selling point for the plane. Boeing had offered Southwest, its biggest Max customer in the U.S., a rebate of $1 million per plane if simulator training was needed when it sold the Dallas-based carrier the aircraft.

Its recommendation that pilots undergo simulator training before the planes — grounded since mid-March after two fatal crashes in a span of five months killed 346 people — could mean airlines will have to wait even longer before they can fly passengers on the fuel-efficient jets, Boeing's best-selling aircraft.

Carriers have lost more than $1 billion in revenue as the grounding wears on, which has forced them to cancel thousands of flights, scrap growth plans and remove the Max from their schedules until at least April. Further delays could continue to raise costs if the grounding continues in the peak summer travel season.

Boeing has developed a software fix for the planes after a flight-control program was implicated in both crashes, but regulators have repeatedly said they have no firm timeline to allow the jets to fly again.

Simulator training could further raise costs for Boeing, which this month plans to shut down production of the planes and last month fired its CEO, Dennis Muilenburg, after a public rebuke from the FAA over the manufacturer's unrealistic timeline for getting the planes back to service.

"The agency will consider Boeing's recommendations for flight crew simulator training during the upcoming Joint Operations Evaluation Board," the Federal Aviation Administration said in a statement, referring to a group of U.S. and international 737 pilots from airlines that operate the Max.

Data from those tests will inform the FAA's official training recommendations, the FAA said.

"The agency will not approve the aircraft for return to service until all parts of the certification process are completed to its satisfaction," it said.

It was not immediately clear how long it would take to train 737 pilots on simulators.

Southwest, which has three simulators with three more on order for the end of the year, said it has "been modeling scenarios for both simulator and computer-based training programs to ensure we're ready to comply with final recommendations and guidance from the federal regulators."