WASHINGTON (Reuters) - Southwest Airlines Co LUV.N said on Thursday it was postponing the return of Boeing Co's BA.N 737 MAX jets to its flight schedule until Feb. 8, the latest delay for the plane involved in two fatal crashes with other carriers over five months.

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The airline had previously planned to keep the jet off its flight schedule through Jan. 5. United Airlines AAL.O and American Airlines AAL.O have canceled flights involving the 737 MAX until January.

"With the timing of the MAX's return-to-service still uncertain, we are extending the MAX-related flight schedule adjustments through Feb. 8, 2020," Southwest said in an update on its website here. "The revision will proactively remove roughly 175 weekday flights from our schedule out of our total peak-day schedule of more than 4,000 daily flights."

On Wednesday, Air Canada AC.T said it had removed its 24 737 MAX planes from flight schedules until Feb. 14.

Boeing said Thursday it had no change in its aim to “return to service in the fourth quarter.” But the plane maker will not hold a certification test flight for the U.S. Federal Aviation Administration (FAA) until early November. If that test flight results in U.S. certification, then European regulators will weigh in, and pilots at airlines must be trained to use the improved system.

This week, Southwest’s pilots union forecast that the grounded jet would return to the skies around February, weeks later than Boeing has projected. No major North American airlines have scheduled commercial flights of the 737 MAX in the fourth quarter.

Southwest is the largest operator of the MAX with 34 jetliners in its fleet. The aircraft was grounded worldwide following a Lion Air crash in Indonesia on Oct. 29, 2018, and an Ethiopian Airlines crash on March 10 that together killed 346 people.

The Southwest Airlines Pilots Association (SWAPA), the union for the airline’s pilots, said flights would likely resume in “probably a February time frame.”

Last week, the Southwest pilots’ union sued Boeing, alleging the planemaker “deliberately misled” the airline and pilots about the 737 MAX. The plane’s grounding in March forced more than 30,000 Southwest Airlines flight cancellations and caused over $100 million in lost wages for pilots, the union said.

Boeing said last week it believes the lawsuit “is meritless and will vigorously defend against it.”

The union noted that following the certification test flight, the European Joint Aviation Authorities Joint Operational Evaluation Board has to submit recommendations to the FAA’s Flight Standards Board.

Airlines will then submit proposed changes to the FAA Certificate Management Office and pilots must complete training.

Boeing is under pressure to deliver updated software and training to regulators for the aircraft to fly again, and the company has been negotiating compensation with customers like Southwest over the financial hit from the grounding.

Southwest, with around 10,000 pilots represented by SWAPA, had 41 more MAX jets on order for this year alone.