Boeing has confirmed that it has been forced to suspend the delivery of some 737MAX aircraft because of a new quality issue that has been detected by a third-party supplier.

The aircraft manufacturer, which only recently beat arch-rival Airbus in the first quarter for deliveries, says the latest issue could affect a “significant” number of undelivered aircraft.

The problem came to light after Spirit AeroSystems, which makes the majority of frames for the 737 model, formally notified Boeing that there was a problem with two of eight fittings that attach the tailfin (vertical stabilizer) to the fuselage.

Spirit says the problem could affect planes built since 2019, although Boeing insists that the issue does not affect the air safety of any 737MAX aircraft currently flying.

Unlike a previous quality issue that paused the delivery of 787 Dreamliners, inspections required for this problem are in a relatively accessible area which should make it much quicker and easier to address any issues.

“This is not an immediate safety of flight issue and the in-service fleet can continue operating safely,” a spokesperson for the beleaguered manufacturer said on Thursday.

“However, the issue will likely affect a significant number of undelivered 737MAX airplanes, both in production and in storage. We expect lower near-term 737MAX deliveries while this required work is completed,” a statement from Boeing continued.

The problem doesn’t affect all 737MAX models, and not every plane manufactured since 2019 has been impacted. The popular MAX8 and 7 are, however, impacted by the problem, along with the high-density MAX8-200 used by Ryan and a military-spec version called the P-8.

Spirit says it is still developing an inspection and repair regime after it identified a “non-standard manufacturing process” that doesn’t conform with FAA standards.

In a statement, the Federal Aviation Administration said that it agrees with Boeing’s assessment that there is no immediate safety issue.

Boeing recently beat European aircraft manufacturer Airbus for quarterly deliveries for the first time in five years. Boeing managed to deliver 130 aircraft in the first three months of 2023, compared to the 127 jets delivered by Airbus.

Key to Boeing’s success in the first quarter was the deliveries of 737MAX jets.