Boeing said this month that workmanship problems with other suppliers might push delivery of the plane a few weeks into 2011. Its statement on Friday said that the engine issue had prompted it to extend the delivery time.

Several analysts said they were puzzled by how little the companies revealed about the problem.

Rolls-Royce and General Electric are both building engines for the Dreamliner. Scott Hamilton, the president of the Leeham Company in Issaquah, Wash., said: “You would think there would be another Rolls-Royce engine or two or three they could use. Why is this one so critical?”

Rolls-Royce said it was working closely with Boeing to expedite its engine deliveries. And on the FlightGlobal Web site, Jon Ostrower suggested that Boeing might be planning to substitute an enhanced version of the Rolls-Royce engine for its basic model in the planes built for All Nippon Airways.

Both Heidi Wood, an analyst at Morgan Stanley, and Richard Aboulafia, an analyst at the Teal Group in Fairfax, Va., said they doubted the first planes would be delivered before next spring.

“Experience shows that programs that ‘swim funny’ do so for much longer than most, especially the bulls, ever expect and can be a grind all the way to the finish,” Ms. Wood wrote in a research note.

Of the nearly 850 Dreamliners on order, more than 200 are scheduled to be equipped with the Rolls-Royce Trent 1000 engine, Boeing said. (Rolls-Royce is a separate company from the carmaker of the same name.) Most airlines have waited patiently for the plane, which is seen as crucial to Boeing’s future. But Boeing could lose some sales and pay additional penalties for further delays.