Southwest Airlines on Thursday again pushed back its target date for returning the Boeing 737 MAX to service because of uncertainty over when regulators will re-certify the plane.

The domestic-oriented US carrier will keep the plane out of service through February 8, 2020, about a month later than the prior timeframe, the company said Thursday on its website.

The planes have been grounded globally since mid-March following two crashes that killed 346 people. Boeing has said it expects to receive regulatory approval to resume flights in the fourth quarter of 2019.

"Southwest Airlines continues to monitor information from Boeing and the Federal Aviation Administration on the impending 737 MAX software enhancements and training requirements," the company said.

Boeing delivered just 63 commercial planes compared to 190 a year ago, due to the grounding of its topselling 737 MAX which continues to dent results
Boeing delivered just 63 commercial planes compared to 190 a year ago, due to the grounding of its topselling 737 MAX which continues to dent results AFP / Mark RALSTON

"We remain confident that, once certified by the FAA, the enhancements will support the safe operation of the MAX."

The move comes as rival US carriers American Airlines and United Airlines in recent days also pushed their target dates for the MAX back to 2020 as the FAA process has dragged on.

Southwest said "proactively" removing the plane from service would "reduce last-minute flight cancellations and unexpected disruptions to our customers' travel plans."

Boeing last week stripped Dennis Muilenburg of his position as chairman at Boeing, although he remains chief executive. The move came hours after a report by international regulators criticized Boeing and the FAA over its certification of the MAX.

Boeing will be under more scrutiny later this month when Muilenburg testifies on Capitol Hill for the first time since the MAX was grounded.