Plane manufacturers Boeing said on Friday that “hairline cracks” had been discovered in the wings of about 40 787 Dreamliners that are currently in production.

The cracks are the latest trouble for the Dreamliner, a high-tech jet largely made of carbon-fiber composite that has been beset with so-called “teething issues” since entering service in 2011, three years behind schedule.

The cracks have not been found on planes that are currently in use, and pose no safety risk, Boeing said, adding that the problem also will not change the firm’s plans to deliver 110 of the 787s this year.

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But the US company said the cracks, which also occurred on the larger 787-9 model currently which is undergoing flight tests, could delay by a few weeks the date when airlines can take delivery of their new planes.

The disclosure raised questions about repair costs and a possible minor increase in the weight of the plane, but did not seem to spell major trouble for Boeing, industry experts said.

The problem arose after the Japanese Wing-maker Mitsubishi Heavy Industries Ltd altered its manufacturing process. Boeing was notified of the issue in February.

“We are discussing with Boeing how to deal with the problem,” a spokesman for Mitsubishi Heavy Industries in Tokyo said. He was unable to comment on why the company changed the manufacturing process.

Boeing, based in Chicago, said its customers had been notified of potential delays, adding that none of the jets potentially affected by the problem had been delivered.

“We are confident that the condition does not exist in the in-service fleet,” Boeing spokesman Doug Alder said.

The US aviation regulator, the Federal Aviation Administration (FAA), said it is aware of the situation.

"The FAA will work with Boeing to ensure that the issues are corrected before the airplanes are delivered," it said in a statement.

Additional reporting by Reuters