(Reuters) – Southwest Airlines Co on Thursday said it was extending the cancellation of Boeing’s 737 MAX planes from its flying schedule until Oct. 1, a day after the Federal Aviation Administration warned it had uncovered a new issue that must be resolved before the plane can be ungrounded.

The airline had previously planned to keep the jet off its flying schedule through Sept. 2. Boeing Co’s MAX fleet has been grounded since March, following a second fatal crash in five months.

Southwest, the world’s largest MAX operator with 34 jets, said the delay will result in removing about 150 flights out of its total peak daily schedule of 4,000.

The FAA on Wednesday said it had identified a new potential risk that Boeing must address on the planes.

Reuters reported on Wednesday that Boeing will not conduct a certification test flight until July 8, under a best-case scenario. The test is a necessary step before Boeing can submit a formal request for approval of a software upgrade for the planes.

Southwest said it “made this decision before any developments of the past few days.”

Once the FAA approves the MAX for flight, Southwest has said it would take about 30 days to get the jets up and running again.

American Airlines said on Thursday it did not “have any schedule announcement to make at this time.” United Airlines on Wednesday said it was extending cancellations into September.

Boeing shares were down 2.5% at $365.48 on Thursday.

(Reporting by David Shepardson in Washington and Ankit Ajmera in Bengaluru; Editing by Saumyadeb Chakrabarty and Bill Berkrot)