The largest customer for Boeing's 787 is predicting another six-month slip in deliveries and has for the first time raised the specter that the new passenger jet's troubles extend beyond production delays to design problems.

International Lease Finance Corp. (ILFC) Chairman Steven Udvar-Hazy told a JPMorgan investor conference that structural design changes have to be made to the 787's center wing box, a move that would require retrofits of the first two flight-test airplanes that are being produced. Calling the state of the program "not pretty," Udvar-Hazy said he doesn't see the 787 making its first flight until this fall and predicts another year will be needed after that for certification. That means the first 787 delivery, to Japan's All Nippon Airways, would be pushed back until the third quarter of 2009, about a year and a half behind the original date of May 2008.

Mr Hazy told a JPMorgan Chase conference that the state of the Dreamliner programme was “not pretty”. He said first deliveries would be delayed for at least another six months because its centre wing box – which holds the wings in place – needed to be redesigned.

Boeing refused to comment on the specifics of the redesign work but said Mr Hazy was not painting an accurate picture of the overall programme. “We are doing some redesign work but things are more complex than what he said,” said Yvonne Leach, for Boeing.

“There’s a whole load of things going on.”