Boeing and a key supplier say that a new manufacturing flaw could delay deliveries of the 737 Max, Boeing’s best-selling plane.

Boeing and Spirit AeroSystems said they discovered improperly drilled fastener holes in the aft pressure bulkhead — which maintains pressure when planes are at cruising altitude – on the fuselages of some models of the 737 Max.

Wichita, Kansas-based Spirit said late Wednesday because it uses multiple suppliers, only some units are impacted and it will continue to deliver fuselages to Boeing.

Boeing said the issue does not affect safety of flight, and 737 Max planes already in service can keep flying.

“This issue will impact near-term 737 deliveries as we conduct inspections to determine the number of airplanes affected, and complete required rework on those airplanes,” Boeing said in a statement. “We continue to deliver 737s that are not affected.”

Aircraft manufacturers have wrestled with supply issues in recent years. Spirit AeroSystems builds the fuselage for Boeing’s 737 Max narrowbody jet and substantial sections of aircraft bodies in other Boeing models. The company builds components for Airbus aircraft as well.

The flaw, in which some fastener holes were found to be elongated, is a setback in Boeing's plan to deliver 400 to 450 Max jets this year, and “some downward revision seems likely,” said Cowen aerospace analyst Cai von Rumohr.

The analyst said that Boeing has delivered only 17 737s in August, compared with 32 in July. Boeing gets about 60% of the plane's purchase price upon delivery, so every delay of 10 deliveries pushes back cash flow by about $300 million — plus the cost of inspections and remedial work — von Rumohr said.

In April, Boeing cautioned that production and delivery of a “significant number” of its 737 Max planes could be delayed because of questions about Spirit AeroSystems' work on the fuselages.

All Max planes were grounded worldwide for nearly two years after a pair of deadly crashes, and production flaws halted deliveries of the larger 787 jetliner several times in the past three years.

Shares of Spirit AeroSystems Holdings Inc. were down 14% in afternoon trading Thursday, while Arlington, Virginia-based Boeing Co.'s stock slipped more than 3%.

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AP Airlines Writer David Koenig in Dallas contributed to this report.