CHICAGO - 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 per cent.

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 conference 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.

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 CEO, 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.'

'In the long term, it's going to put the break-even point for the 787 probably a little further out and it's going to increase the cost of development,' Mr Hamilton said.

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

'It's really unclear at this point what the impact is,' said Craig Fraser, aerospace and defence analyst at Fitch Ratings.

'If this is just a temporary delay of a few weeks, it's probably not much of an issue for the program or financially,' he said. 'If it turns out to be a modification of the airplane that takes several months, it could be costly.' -- REUTERS