Boeing probably won't deliver its first 787 until July next year at the earliest, a Goldman Sachs analyst said on Friday, potentially adding another six month delay onto the troubled program, which is already nine months behind schedule.

Airlines are still enthusiastic about Boeing's new carbon-composite plane, which promises to cut fuel costs by 20 percent, but some are starting to show impatience with slow progress on the aircraft, which was originally set to enter service in May this year.

"We now think deliveries will start in the third quarter of 2009 versus the current 'early' 2009 target," wrote Goldman Sachs analyst Richard Safran in a report issued on Friday.

"Boeing continues to underestimate the amount of work required on the 787," said Safran, citing unnamed sources familiar with the program.

He said production problems on the first batch of 787s on Boeing's production line would delay turning the power on in the first plane, push back the first test flight and extend the amount of time Boeing needs to conduct full flight tests on the aircraft.

Boeing has been updating investors on the progress of the plane each quarter, but generally does not make comments on specific development issues.

Wall Street analysts and aviation bloggers accurately predicted the previous two delays on Boeing's 787, long before the company admitted to problems on the aircraft.