Boeing Co. found more mistakes with holes drilled in the fuselage of its 737 Max jet, a setback that could further slow deliveries on a critical program already restricted by regulators over quality lapses.

The latest manufacturing slip originated with a supplier and will require rework on about 50 undelivered 737 jets to repair the faulty rivet holes, Boeing commercial chief Stan Deal said in a note to staff.

While he didn’t identify the contractor, a spokesman for fuselage supplier Spirit AeroSystems Holdings Inc. said it’s aware of the issue and will conduct repairs.

The extra time required for inspections and repair work could delay near-term plane deliveries, Deal said in his memo, which was seen by Bloomberg News. He didn’t say whether any action would be required on the in-service 737 fleet.

“This is the only course of action given our commitment to deliver perfect airplanes every time,” Deal said in his note.

The defect follows a string of manufacturing lapses at Boeing, including a near-catastrophic panel blowout on an Alaska Airlines 737 Max last month. The Federal Aviation Administration has stepped up scrutiny of Boeing’s manufacturing and supplier systems and has capped 737 production until quality improves.

The problem disclosed Sunday is the latest in a series of glitches originating with Boeing’s former aerostructures unit. A drilling fault on an aft pressure bulkhead supplied by Spirit Aero slowed deliveries of the 737 Max last year, the planemaker’s most important generator of cash flow. A separate issue with tail-fin fittings affected output earlier in 2023.

In the latest instance, Deal said a worker at a Boeing supplier flagged that two holes in the plane’s fuselage may not exactly meet specifications. The problem “is not an immediate flight safety issue and all 737s can continue operating safely,” he said.

Still, he said many employees have expressed frustration at how unfinished work, either by suppliers or within Boeing’s factories, can ripple through aircraft production lines. To address this, Boeing has recently told a major supplier to hold shipments until all work has been properly completed, he said.

“While this delay in shipment will affect our production schedule, it will improve overall quality and stability,” Deal said.