Boeing acknowledged on Friday that it may not be able to deliver its 787 as fast as hoped.

The company said it has told customers expecting the next 787 deliveries that those planes have either been delayed, or at risk for a delay.

Boeing is still building the long-range, fuel-efficient planes, and it said on Friday that it has no plans to slow production.

Norwegian Air Shuttle confirmed that it got such a warning from Boeing. The Oslo-based budget airlines spokesman, Lasse Sandaker-Nielsen, said that delivery of the planes, scheduled to be flying for the airline in May or June, might not be possible. He gave no reason for the delays.

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Norwegian is one of Europes fastest-growing airlines and had planned to begin its first long-haul flights to New York and Bangkok with the Boeing 787s.

The worlds fleet of 50 787s has been grounded since Jan. 16. Boeing and investigators are trying to figure out why one aircraft battery caught fire and another one smouldered and forced an emergency landing.

The National Transportation Safety Board is still probably weeks away from determining the root cause of the Jan. 7 battery fire in a Japan Airlines 787 parked at Bostons Logan International Airport, NTSB Chairwoman Deborah Hersman told reporters this week. In about 30 days the board plans to issue an interim report on its investigation and post online all the test results and factual information gathered in its investigation, Hersman said.

The 787 is the first commercial airliner to rely heavily on lithium-ion batteries. Each plane has two of the 63-pound blue power bricks, one near the front to provide power to the cockpit if the engines stop, and one near the back to start up the auxiliary power unit, which is essentially a backup generator.

One of the big unknowns right now is whether Boeing will need to dro the lithium-ion batteries in favour of a battery more like that used on other planes.

The NTSBs detailed updat on Thursday said the fire on the Boston plane started when one of the batterys eight cells short-circuited, causing it to heat up. The heat in turn caused swelling in neighbouring cells, and they short-circuited, too.

The focus on the batterys charging, design or manufacturing process seems to limit the risk of a larger problem and is somewhat of a positive development for Boeing,Jefferies analyst Howard A. Rubel wrote in a note on Friday. We believe the risk of a major re-design has declined.