Boeing on Tuesday postponed the first test flight of its 787 Dreamliner for a fifth time, citing a structural problem that marked the latest blow to the credibility of the No. 2 planemaker and sent its shares tumbling 9 percent.

Boeing gave no new date for the flight or the first delivery, which also would be rescheduled. The revolutionary carbon-composite aircraft, already two years behind its original schedule, was to fly in the second quarter of 2009.

Obviously we are all anxious to see this airplane fly, but it's important that it flies when it's ready to fly and when we have high confidence in what we can accomplish with it in flight tests, said Scott Carson, chief executive of Boeing Commercial Airplanes, on a call with analysts and reporters.

Boeing said it delayed the test flight to reinforce an area within the side-of-body section of the aircraft. The fix itself is relatively inexpensive and does not raise broader concerns about the aircraft, the company said.

It's a setback that's going to disappoint investors. Boeing's shares will suffer a near-term setback, said Jack Ablin, chief investment officer at Harris Private Bank.

I'm hopeful that this issue can be resolved quickly, Ablin said. But my sense is that if it's big enough to delay the introduction, then this is a serious concern.

Boeing shares were down 7.31 percent at $43.47 on the New York Stock Exchange in afternoon trading, off an earlier low at $42.52.

The date of the first test flight has been delayed repeatedly because of production problems and a two-month labor strike. The latest postponement could challenge the credibility of the company and its chief executive, James McNerney, who said as recently as May that the plane would fly in June.

I am a little surprised, said Alex Hamilton, an analyst at Jesup & Lamont Securities. I think a lot of confidence has been blown.

Chicago-based Boeing said its financial outlook would be updated in July to reflect any impact from the new delay.

If this is just a temporary delay of a few weeks, it's probably not much of an issue for the program or financially, said Craig Fraser, aerospace analyst at Fitch Ratings. If it turns out to be a modification of the airplane that takes several months, it could be costly.

This latest setback came on a day that rival Airbus celebrated the delivery of an Airbus assembled in China, the first outside Europe. However, its equivalent plane to the 787 - the A350 - is not due until the middle of next decade.

DELAYS AND DISAPPOINTMENT

Boeing and Airbus are being hit hard as passenger airlines and cargo operators grapple with economic woes around the world. Boeing has seen a net 45 cancellations for Dreamliner orders this year.

When it is completed, the 787 promises to usher in an era of lighter, more fuel-efficient planes. And with oil near $70 a barrel, airlines are desperate for lower fuel bills.

In early June, Boeing said it had more than 50 customers that have placed orders for more than 850 of the airplanes.

It is disappointing that Boeing put off the first flight of 787, said a spokesman for Japan's All Nippon Airways Co, which is the first customer set to receive 787s. We will urge Boeing to clarify the entire schedule regarding 787 as soon as possible.

Boeing had originally planned to fly the Dreamliner in the summer of 2007 with the first delivery set for 2008.

We are disappointed, yet we remain committed to the 787 and continue to believe that it will be a game-changer when it comes on the scene, said Continental spokeswoman Julie King.

(Additional reporting by Karen Jacobs, Bill Rigby, Deepa Seetharaman and Nobuhiro Kubo; Editing by Lisa Von Ahn, Maureen Bavdek, Matthew Lewis and Steve Orlofsky)