A new software issue has been identified in the troubled Boeing 737 Max that is almost certain to further delay the plane’s return to flying after two deadly crashes.

Boeing said US regulator the Federal Aviation Administration (FAA) had identified an “additional requirement” for software changes which the aircraft manufacturer has been working following the first crash.

A spokesman said: “Boeing agrees with the FAA’s decision and request and is working on the required software to address the request.”

Government test pilots trying out Boeing’s updated Max software in a flight simulator last week found a flaw that could result in the plane’s nose pitching down, according to sources.

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In both Max crashes, the plane’s flight-control software pushed the nose down based on faulty readings from a sensor.

The control system has been installed to automatically prevent a plane from stalling.

In a statement, the FAA said it would lift its grounding of the plane only when it deemed the jet safe.

The agency said: “On the most recent issue, the FAA’s process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate.”

The Max began passenger flights in 2017 and is Boeing’s best-selling plane, although fewer than 400 have been delivered to airlines.

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A Max flown by Indonesia’s Lion Air crashed in October, and an Ethiopian Airlines Max crashed in March. In all, 346 people died.

Days after the second crash, regulators around the world grounded the plane.

As a result Boeing is scaling back the power of flight-control software called MCAS which pushes the nose down in certain circumstances.

It is also linking the software’s nose-down command to two sensors instead of relying on just one in the original design.

It is still uncertain what kind of training pilots will get for flying the plane with the new software.

Preliminary reports into both accidents have suggested they were triggered by a flight control system deploying at the wrong time, due to a faulty sensor.

Last month, the FAA indicated that approval of Boeing’s changes to the 737 Max could come in late June. That would have allowed test flights in early July.

There were initial hopes among airlines that the 737 Max would be back in the air during the summer, but that timetable has now been pushed back to later this year.

The FAA has been criticised for its lack of oversight and the certification process that cleared the Max to fly.

Captain Chesley Sullenberger told a Congressional hearing into the 737 Max that the crashes were “demonstrable evidence that our current system of design and certification has failed us”.