Boeing on Thursday warned it will likely have to reduce deliveries of its 737 Max airplane because of a problem with a part made by supplier Spirit AeroSystems.

Boeing said its supplier informed the company a “non-standard” manufacturing process was used on two fittings in aft fuselages. It said the issue affects certain 737 Max 8 planes, the company’s most popular model, with customers including American Airlines and Southwest Airlines. It also affects certain 737 Max 7, the 737 8200 and P-8 planes.

Boeing said the problem was not an “immediate safety of flight issue and the in-service fleet can continue operating safely.”

However, the issue will likely affect a significant number of undelivered 737 MAX airplanes, both in production and in storage,” the manufacturer said in a statement.

The problem, the most recent in a string of production issues, hits Boeing as it scrambles to increase production and deliveries of its best-selling plane while customers await new jetliners to capitalize on a rebound in travel. 

Shares of Boeing fell 4% in after-hours trading on Thursday after it disclosed the problem. Shares of Spirit AeroSystems fell nearly 8%.

Spirit manufacturers some of the fuselages used in Boeing jets and said in a statement it notified Boeing of a “quality issue” with certain 737 models.

“Spirit is working to develop an inspection and repair for the affected fuselages. We continue to coordinate closely with our customer to resolve this matter and minimize impacts while maintaining our focus on safety,” the company said.

Boeing has notified the Federal Aviation Administration of the issue and is working to inspect and address the fuselages as needed, the company said.

“We expect lower near-term 737 MAX deliveries while this required work is completed. We regret the impact that this issue will have on affected customers and are in contact with them concerning their delivery schedule,” Boeing said in a statement. “We will provide additional information in the days and weeks ahead as we better understand the delivery impacts.”

The FAA didn’t immediately respond to request for comment.

It’s the latest production problem for Boeing and its customers. Boeing earlier this year paused deliveries of its 787 Dreamliners for several weeks to address a data analysis flaw, and in 2021 and 2022 it struggled with other production flaws on the wide-body jets that halted deliveries for months.

The company on Tuesday reported March deliveries of 64 planes, the highest tally since December, amid an industry-wide shortage of new jets.

Airline executives have cited aircraft supply constraints as among the chief challenges in ramping up flying ahead of the peak travel season.

“We’re aware of the issue and working with Boeing to understand how it may impact our MAX deliveries,” an American Airlines spokesman said in statement.

— CNBC’s Leslie Josephs and Phil LeBeau contributed to this report.