Boeing is planning to submit a proposed software upgrade for the grounded 737 Max in coming weeks, it has said.
The aircraft maker on Monday confirmed a statement from the Federal Aviation Administration that it would be late in submitting the upgrade after promising to deliver a fix for government approval by last week.
"We are working to demonstrate that we have identified and appropriately addressed all certification requirements and will be submitting it for FAA review once completed in the coming weeks," Boeing said.
"We will take a thorough and methodical approach to the development and testing of the update to ensure we take the time to get it right."
More than 300 Boeing 737 Max jets have been grounded worldwide after two crashes – in Indonesia in October and in Ethiopia last month – killed nearly 350 people.
Boeing, facing one of its worst crises in years, is under pressure from victims' families, airlines, US legislators and regulators around the world to prove that the automated flight control systems of its 737 Max aircraft are safe and that pilots have the training required to override them in an emergency.
Southwest Airlines said on Monday that it was "publishing a revised schedule for April and May that is built around the currently available Southwest fleet and intends to reduce drastically last-minute trip disruptions and same-day cancellations".
Earlier on Monday, FAA spokesman Greg Martin said: "Time is needed for additional work by Boeing as the result of an ongoing review of the 737 Max Flight Control System to ensure that Boeing has identified and appropriately addressed all pertinent issues."
The authority's acting administrator, Dan Elwell, said last week that the agency would not return the planes to service until its analysis "of the facts and technical data indicate that it is appropriate".
Boeing said last week that it had reprogrammed software on its 737 Max passenger jet to prevent erroneous data from triggering an anti-stall system that is under increasing scrutiny after the two deadly nose-down crashes.
The world’s largest planemaker said the anti-stall system, which is believed to have repeatedly forced the nose lower in at least the Indonesian accident, would only do so one time after sensing a problem, giving pilots more control.
It would also be disabled if two airflow sensors that measure the angle of the wing to the airflow, a fundamental parameter of flight, offered widely different readings, Boeing said last week.
Federal prosecutors, the transport department's inspector general and US legislators are investigating the authority's certification of the 737 Max.
Transport secretary Elaine Chao said she was naming an outside panel to review the issue.
