Boeing said it had paused test flights of the 777-9, a large plane whose development had already been delayed by years, after discovering problems with a key part.

The airplane manufacturer discovered the problem with a structural component that connects the engine to the plane’s body, during routine maintenance on one of four test planes. None of the three other planes had flights scheduled. Each engine includes a redundant version of the part, which is custom made for the 777-9, according to Boeing.

The company is replacing the part, investigating the issue on each plane and “will resume flight testing when ready,” Boeing said.

The pause represents the latest blow to development of the 777X series of planes, starting with the 777-9. Boeing operated its first 777-9 flight in January 2020, saying at the time that it expected deliveries to begin the following year.

The 777-9 is a large, twin-aisle plane designed for long-distance, international flights. It is capable of carrying around 426 passengers over a distance of more than 7,200 nautical miles. Boeing plans to later build a slightly smaller version of the plane capable of traveling further, the 777-8, and a cargo version, the 777-8 Freighter.

Boeing did not say how long it expected the pause in test flights to last or whether it would delay its first delivery of the 777-9, which the company expects to happen next year. The Federal Aviation Administration said that Boeing notified the agency about the problem last week, and the company said it planned to keep the F.A.A. updated on its findings. The pause in test flights was reported earlier by The Air Current, an aviation news service.

Boeing began flights of the model for F.A.A. certification in July, though it had been testing the plane on its own for some time. The test fleet had already carried out more than 1,200 flights in various regions and conditions as of the end of last month, Dave Calhoun, the company’s former chief executive, said on a call with investor analysts at the time.

Mr. Calhoun stepped down this month and was succeeded by Kelly Ortberg, the former chief executive of Rockwell Collins, a major supplier to Boeing and its main rival, Airbus. Mr. Calhoun’s departure had been announced after a harrowing episode in January in which a panel of a Boeing 737 Max blew off during an Alaska Airlines flight.

There were no major injuries from that event, but it and an earlier crisis involving two fatal 737 Max crashes five years ago have slowed production at Boeing and contributed to the widening gap between the company and Airbus.