In a letter shared with the media, Boeing Commercial Airplanes chief executive Stan Deal told staff a worker at a supplier had found misdrilled holes in fuselages.

Spirit AeroSystems, based in Wichita, Kansas, makes many of the fuselages for Boeing MAX jets.

Mr Deal wrote: "While this potential condition is not an immediate safety issue and all 737s can continue operating safely, we currently believe we will have to perform rework on about 50 undelivered planes."

The employee at the fuselages supplier told his manager two holes may not have been drilled according to specifications, Mr Deal added.

Spirit spokesperson Joe Buccino told the Reuters news agency: "We are in close communication with Boeing on this matter."

It comes after more than 170 planes were grounded by US regulators when an Alaska Airlines 737 MAX 9 was forced to make an emergency landing on 5 January.

In that incident a window and chunk of fuselage blew out of the side of the plane shortly after taking off from Portland, Oregon.

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Loose hardware in door plugs

Alaska Airlines and United Airlines, the only other US airline flying the MAX 9, reported finding loose hardware in door plugs of other planes they inspected after the incident at the start of the year.

The Federal Aviation Administration grounded all MAX 9s in the US the day after the blowout - two weeks later, the agency approved the inspection and maintenance process to allow the planes to return to flying.

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Alaska Airlines and United Airlines have begun returning some to service.

Boeing, based in Washington, said last week it was withdrawing a request for a safety exemption needed to certify a new, smaller model of the 737 MAX airliner.