Gordon Johndroe, a Boeing spokesman, said the company agreed with the FAA finding and was addressing the issue as well as a broader software redesign that's been underway for eight months.

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"Addressing this condition will reduce pilot workload" by making it easier to respond to an uncommanded stabiliser motion, Johndroe said.

"The safety of our airplanes is Boeing's highest priority," he added. "We are working closely with the FAA to safely return the MAX to service."

The Lion Air crash in Indonesia in October and the Ethiopia Airlines disaster outside Addis Ababa in March triggered the grounding of all MAX aircraft worldwide on March 13 as the cockpit software linked to both crashes is being updated for approval by regulators.

"The FAA will lift the aircraft's prohibition order when we deem it is safe to do so," the FAA said in the statement.

Exercises on a Boeing 737 MAX simulator in recent days revealed that pilots might have difficulty responding to the newly identified failure, the person said.

Just as MCAS uses a motor to move a small wing at the tail of the plane to lower the nose, the latest issue could prompt that same wing to move without pilot commands. The tail wing is known as a horizontal stabiliser.

The motor on the stabiliser adjusts up and down movements known as trim. Examining how trim failures occur has been a central part of safety reviews of the plane because it was central to the accidents.

The FAA as well as an independent Technical Advisory Board have been reviewing Boeing's software fix in multiple sessions in a special Boeing simulator that is designed for engineering reviews.

"We continue to evaluate Boeing's software modification to the MCAS and we are still developing necessary training requirements," the agency said in its statement.

Boeing hasn't presented its final proposed fix to FAA for approval. Before it can do so, it has to conduct a final test flight with FAA pilots.

Bloomberg