Australian flag carrier Qantas said on Thursday it had grounded one Boeing 737NG due to a structural crack, and was urgently inspecting 32 others for the flaw.
The grounding is the latest safety concern for Boeing, as it reels from two 737 MAX crashes that killed 346 people and highlighted problems with the planes' flight handling software.
The US aviation authority this month ordered checks of Boeing 737NG planes that had flown more than 30 000 times.
That came after the company reported the "pickle fork", which helps connect the wing to the fuselage, could be prone to cracking.
Qantas said on Thursday it had found the fault in a more lightly used aircraft, one that had recorded fewer than 27 000 flights.
"This aircraft has been removed from service for repair," Qantas said in a statement, adding it had hastened its inspections of 32 other 737NG plans to be completed by Friday.
The announcement by Qantas raised fears the cracking issue could affect newer planes than previously thought, leading to calls for the Australian airline to ground its entire 737 fleet.
"These aircraft should be kept safe on the ground until urgent inspections are completed," an engineers' union representative, Steve Purvinas, said in a statement.
However Qantas described the call to ground its 737 fleet as "completely irresponsible".
"We would never operate an aircraft unless it was completely safe to do so," Qantas head of engineering Chris Snook said.
"Even when a crack is present, it does not immediately compromise the safety of the aircraft."
The FAA initially said Boeing notified the agency of the problem after encountering the issue on a plane in China and that subsequent inspections showed other planes also had cracks.
The NG is a precursor plane to the Boeing 737 MAX, which has been grounded since mid-March following the two deadly crashes in Indonesia and Ethiopia.
Boeing continues to come under fire
Boeing chief executive Dennis Muilenburg faced another round of tough questions on Wednesday from US lawmakers who accused the company of a "lack of candour" over the crashes.
In his first appearance before Congress since the 737 MAX was grounded in March, Muilenburg apologised for the crashes and acknowledged shortcomings, but broadly defended Boeing's development of the ill-fated aircraft.
Senators from both parties signaled clear dissatisfaction, bordering on rage in some cases.
"Boeing is the company that built the flying fortress that saved Europe," said Democratic Senator Tammy Duckworth, a former National Guard helicopter pilot who lost both legs during the Iraq War.
"You have told this committee and you told me half-truths over and over again," said Duckworth, who represents Illinois, home to Boeing's corporate headquarters.
"You have not told us the whole truth and these families are suffering because of it."
Muilenburg stuck to the company's longstanding stance that development of the MAX followed time-tested company procedures and defended it against charges that it cut corners on safety and was too cozy with regulators the Federal Aviation Administration.
Many analysts view the hearings as a can't-win situation for Muilenburg and expect him to exit the company in the foreseeable future, most likely after the MAX returns to service.