Boeing is putting the brakes on deliveries of some 737 MAX jets as it grapples with a new supplier quality problem.

The latest announcement comes after the planemaker said last month that it was getting ready to ramp up production.

The problem involves the installation of two fittings that join the aft fuselage made by Spirit AeroSystems to the vertical tail, which were not attached correctly to the structure of the fuselage before it was sent to Boeing.

The problem could stretch back to 2019.

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Shares of Boeing fell 5% in extended trading, while shares of Spirit AeroSystems fell more than 11%.

A "significant" number of undelivered 737 MAX airplanes both in production and in storage could be impacted.

It also could result in lowered 737 MAX deliveries in the near term, the company said.

FOX Business has reached out to Boeing for comment.

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Boeing said in late March it was planning to ramp up production for the 737 MAX airliners "very soon." 

The problem impacts a portion of the 737 MAX family of airplanes, including the MAX 7, MAX 8 and MAX 8200 airplanes as well as the P-8 Poseidon maritime surveillance aircraft.

Boeing said it is not a safety of flight issue and in-service planes can continue to operate.

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The Federal Aviation Administration said it had "validated" Boeing's assessment that there was no immediate safety issue.

Certain versions of the aircraft, like the MAX 9, use fittings from different suppliers and were correctly installed.

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Boeing had aimed to increase monthly MAX production rates from 31 to 38 by June.

Reuters contributed to this report.