Boeing said it is working diligently to complete United Airlines' next 787s for delivery but that the process is "taking a few days longer than anticipated."

The delay, while short, is another missed date for the 787 program which was delayed for nearly 3-1/2 years to address quality problems with the jet that uses substantial amounts of lightweight composite materials to replace aluminium in the fuselage and wings.

United took delivery of its first 787 in September and is due to start carrying customers with it on November 4. The airline is scheduled to receive five of the jets this year.

United said on Tuesday that Boeing told it the second plane was not ready for delivery and the airline said it believes all of the four remaining jets could be delayed.

It has begun notifying customers who were scheduled to fly on the second 787 jet that they will travel on a different model of plane instead.

"We're offering to refund or re-book customers who specifically intended to fly on one of the early Dreamliner flights," said Christen David, director of corporate communications for United Continental Holdings Inc.

"We believe this year's subsequent 787 deliveries could be delayed as well, but we are hopeful that we will still receive four more 787s this year," said David.

She declined to say what issue had delayed the plane delivery, or when it was due to be delivered now.

Boeing said it was not one specific issue with the plane and declined to elaborate. It said the issues would not affect delivery of 787s to other customers.

"The process for completing an airplane requires thoroughness and a disciplined adherence to process," said Tim Bader, a Boeing spokesman. "We've laid out a challenging schedule for the team. But we've also told them we cannot compromise on the rigor with which we finish our work, test our products and certify them for delivery."

The 787 Dreamliner, a wide-body jet, seats 219 passengers in United's configuration, and is billed as Boeing most fuel-efficient jet. It was initially scheduled to enter service in May 2008, but delays pushed its first flight back to December 2009 and it entered service on October 26, 2011, with launch customer All Nippon Airways (9202.T). (Reporting by Alwyn Scott; Editing by Tim Dobbyn)

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