Southwest Airlines Boeing 737 MAX aircraft parked in Victorville, California

Southwest Airlines announced Thursday it will once again delay resumption of Boeing 737 MAX flights until June 6, because of continued uncertainty over the troubled aircraft's return to service.

The airline thereby joins American Airlines and United Airlines which also have taken the 737 MAX out of their until June 4.

"By proactively removing the MAX from scheduled service, we can reduce last-minute flight cancellations and unexpected disruptions to our customers' travel plans," Southwest said.

Southwest has 34 737 MAX's in its fleet, the most of any airline.

The delay affects about 330 weekday flights, or about eight percent of the airline's total flights on peak travel days, it said, promising to transfer concerned customers to other flights.

"We offer our apologies to our customers impacted by this change, and we thank them for their continued patience," the company said.

The 737 MAX was grounded worldwide March 13, 2019 after two crashes claimed the lives of 346 people.

Southwest, like American Airlines and United, had delayed the date for reintegrating the aircraft into its flight program several times before, but had hoped to resume flights with the 737 Max on April 14.