Boeing (BA) said deliveries that had been scheduled to begin next May will be pushed back to late November or December 2008.
The first flight, already pushed back once from the initial target of earlier this fall, now is anticipated around the end of the first quarter of 2008.
Boeing said the postponement will not materially affect its earnings or guidance for next year.
"We are disappointed over the schedule changes that we are announcing today," said Chairman and CEO Jim McNerney. "Notwithstanding the challenges that we are experiencing in bringing forward this game-changing product, we remain confident in the design of the 787, and in the fundamental innovation and technologies that underpin it."
McNerney had publicly voiced confidence as recently as four weeks ago that the airplane maker would be able to deliver the first 787 on time next May to Japan's All Nippon Airways, despite skepticism among industry observers following the first postponement.
On Sept. 5, Boeing formally pushed back the first test flight to mid-November or mid-December due to complications with final assembly and finalizing flight-control software. That would have left the company just five to six months before the first delivery, or about half the time it took to test the 777 a decade ago.
The company first acknowledged problems meeting the original test-flight schedule in August when it cited ongoing challenges with out-of-sequence production work, including parts shortages, and remaining software and systems integration activities.
Boeing shares, which had moved higher Wednesday before the late-morning announcement, quickly tumbled $2.95, or 2.9%, to $98.50 in midday trading.