FILE PHOTO: A number of grounded Southwest Airlines Boeing 737 MAX 8 aircraft are shown parked at Victorville Airport in Victorville, California, U.S., March 26, 2019. REUTERS/Mike Blake/File Photo

CHICAGO (Reuters) - Southwest Airlines Co LUV.N said on Tuesday it was cancelling nearly double the number of flights through April than it has done since the Boeing 737 MAX grounding last March amid uncertainty over when the aircraft will fly again.

Boeing Co's BA.N 737 MAX was banned in March after two deadly crashes in Indonesia and Ethiopia. Last week, the U.S. Federal Aviation Administration said it would not approve the aircraft's return to service before 2020, prompting Boeing to freeze 737 production for the first time in two decades.

The production halt itself does not immediately impact airlines but the deepening crisis at Boeing is crimping the growth plans of top customers like Southwest, which are still awaiting deliveries of hundreds of 737 MAX planes that have already been built.

The low-cost carrier said on Tuesday it was removing roughly 300 weekday flights from its total peak-day schedule of more than 4,000 daily flights through April 13, nearly double the amount it had canceled through March 6.

Southwest, the world’s largest Boeing 737 MAX operator, had 34 MAX planes in its fleet when they were grounded in March and was hoping to take delivery of up to 41 more jets by early next year, allowing it to restore frequencies and routes it has been forced to cancel due to a shortage of aircraft.

Among the other U.S. 737 MAX carriers, American Airlines Group Inc AAL.O last week extended its 737 MAX cancellations through April 6, while United Airlines Holdings Inc UAL.O is scheduling without the plane until early March.

Airlines have said they will need at least one month after FAA approval to prepare their jets and pilots with software updates that Boeing has developed to address issues that played a role in both crashes.