Boeing Co said on Sunday it will have to do more work on about 50 undelivered 737 Max airplanes, potentially delaying near-term deliveries, after its supplier Spirit AeroSystems discovered two mis-drilled holes on some fuselages.

Boeing confirmed the findings in response to a Reuters query after industry sources said a spacing problem had been discovered in holes drilled on a window frame, but the jetmaker said safety was unaffected and existing 737s could keep flying.