The US Federal Aviation Administration now expects to receive Boeing's final 737 Max software update in the "coming weeks", saying Boeing needs more time to develop the fix.
"The FAA expects to receive Boeing's final package of its software enhancement in the coming weeks for FAA approval," the agency says in a 1 April statement. "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."
Once received, the FAA will subject the updated software to "a rigorous safety review".
"The FAA will not approve the software for installation until the agency is satisfied with the submission," it adds.
In a statement, Boeing says it "continues to work with the US Federal Aviation Administration and other regulatory agencies worldwide on the development and certification of the MCAS software update and training programme".
"We are working to demonstrate that we have identified and appropriately addressed all certification requirements and will be submitting for FAA review once completed in the coming weeks," the company adds. "Safety is our first priority, and 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".
Last week, Boeing said it had developed a software change that prevents the 737 Max's manoeuvring characteristics augmentation system (MCAS) from triggering as the result of faulty aircraft sensor data.
The October 2018 crash of a Lion Air Boeing 737 Max 8 occurred after MCAS triggered repeatedly, seemingly due to faulty angle-of-attack sensor information. Officials have said the March crash of an Ethiopian Airlines 737 Max 8 shows similarities to the Lion Air crash.
Boeing's software changes makes MCAS rely on two AOA indicators (the existing system relies on one). The upgraded MCAS also monitors the sensors for variations, and deactivates if necessary.
Additionally, the software tweaks prevent MCAS from moving the aircraft's horizontal stabiliser to a degree that pilots cannot overcome by pulling back on the control column, Boeing says.
Story updated on 1 April to include comments from Boeing.