In an update on its work following the incident on an Alaska Airlines MAX 9 on 5 January, the Federal Aviation Administration (FAA) said it had banned Boeing from expanding production of all its best-selling 737 MAX model variants.

It blamed "unacceptable" quality issues for the decision, which aviation experts said was a first for the industry.

The penalty threatens to further deepen delays in airlines, such as major customer Ryanair, receiving already delayed orders for MAX planes.

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Europe's largest airline by passenger numbers does not utilise the MAX 9 but is already being hampered by Boeing not meeting delivery deadlines for MAX 8 and MAX 10 models.

The move will inflict further damage not only on Boeing, a company that had been seeking to restore confidence in its fleet of aircraft following two fatal crashes in 2018 and 2019, but its suppliers too.

Loose bolts were found on multiple MAX 9 aircraft that had an emergency exit door panel replacement.

The bolts appear to have been the focus of the investigation, which was widened to cover quality issues more generally at Boeing.

Ryanair's boss, Michael O'Leary, told Sky News last week that while it had seen evidence of poor work at Boeing, the airline's planes were safe.

FAA Administrator Mike Whitaker said: "We will not agree to any request from Boeing for an expansion in production or approve additional production lines for the 737 MAX until we are satisfied that the quality control issues uncovered during this process are resolved.

"The quality assurance issues we have seen are unacceptable."

Crucially, no time restriction was placed on the order.

Boeing's latest 737 master schedule, which sets the production pace for suppliers, called for production to rise to 42 jets per month in February, 47.2 in August, 52.5 by February 2025 and 57.7 in October 2025.

Boeing said it would continue to cooperate "fully and transparently" with the FAA and follow the agency's direction as it took action to strengthen safety and quality.

Its shares fell 2% in after-hours trading on Wednesday.

Alaska and United Airlines, which had been forced to cancel thousands of flights between them while their MAX 9s were checked, aim to complete that process in the coming days and return the planes to service from Friday and Sunday respectively.