By Tim Hepher and Valerie Insinna

(Reuters) -Boeing Co said on Sunday it will have to do more work on about 50 undelivered 737 MAX airplanes, potentially delaying some 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 an "edge margin", or spacing problem, had been found in holes drilled on a window frame on some jets.

Boeing, which has been under fire from regulators and airlines since the Jan. 5 blowout of a door plug on a 737 MAX 9, said safety was unaffected and existing 737s could keep flying.

"This past Thursday, a supplier notified us of a non-conformance in some 737 fuselages. I want to thank an employee at the supplier who flagged to his manager that two holes may not have been drilled exactly to our requirements," Boeing Commercial Airplanes CEO Stan Deal said in a letter to staff referring to Spirit, which is the sole 737 fuselage supplier.

"While this potential condition is not an immediate flight safety issue and all 737s can continue operating safely, we currently believe we will have to perform rework on about 50 undelivered airplanes," Deal said in the letter, first reported exclusively by Reuters.

Spirit spokesperson Joe Buccino told Reuters that as part of its 360-degree quality management program, a member of its team identified an issue that did not conform to engineering standards.

"We are in close communication with Boeing on this matter," he said.

Deal said Boeing plans to devote several "factory days" this week at the Renton 737 plant outside Seattle to work on the mis-aligned holes and finish off other outstanding work. Such days allow teams to pause normal work and attend to specific tasks without shutting production.

The amount of rework time is expected to be finalised in coming days.

It is the latest effort by Boeing to tighten its operations after the blowout on an Alaska Airlines jet threw the spotlight on quality controls.

Investigators, who have been examining whether the bolts on the Alaska Airlines door plug were missing or badly fitted, are expected to issue an interim report this week.

At the same time, Boeing has asked a major supplier, which it did not identify, to halt shipments until jobs have been completed to specification, Deal said.

"While this delay in shipment will affect our production schedule, it will improve overall quality and stability."

Boeing said parts that already conform to the right specification can continue to be shipped.