By Alwyn Scott and Tim Hepher

NEW YORK, March 7 (Reuters) - Boeing Co said on Friday that "hairline cracks" had been discovered in the wings of about 40 787 Dreamliners that are in production, marking another setback for the company's newest jet.

The cracks have not been found on planes that are in use by airlines and therefore pose no safety risk, Boeing said, adding the problem also will not alter Boeing's plans to deliver 110 787s this year.

However, Boeing said the cracks, which also occurred on the larger 787-9 model currently 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.

Wing-maker Mitsubishi Heavy Industries Ltd notified Boeing in February of the problem, which arose after the Japanese company altered its manufacturing process.

"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 it immediately notified customers of potential delays. It said none of the jets potentially affected by the problem has been delivered.

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

The U.S. regulator 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," the Federal Aviation Administration said in a statement.

Boeing shares fell 54 cents, or 0.4 percent, to $128.00 in after-hours trading.

TEETHING ISSUES

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.

Last year, lithium-ion batteries overheated on two Dreamliners, prompting regulators to ground the worldwide fleet for more than three months while Boeing redesigned the battery system. Another battery overheated this year.

Airbus also has struggled with wing cracks on its A380 jet.

"If they can keep the delivery schedule going, it shouldn't be a major problem for customers," said Richard Aboulafia, aerospace analyst at the Teal Group in Fairfax, Virginia.

"But there is an expense."

He added that the manufacturing change was probably aimed at reducing the weight of the plane, as was the case with the A380.