Plane bodies made by Boeing’s largest supplier regularly left the factory with serious defects, according to a former quality inspector at the firm.

Santiago Paredes who worked for Spirit AeroSystems in Kansas, told the BBC he often found up to 200 defects on parts being readied for shipping to Boeing.

He was nicknamed “showstopper” for slowing down production when he tried to tackle his concerns, he claimed.

Spirit said it “strongly disagree[d]” with the allegations.

“We are vigorously defending against his claims,” said a spokesperson for Spirit, which remains Boeing’s largest supplier.

Mr Paredes made the allegations against Spirit in an exclusive interview with the BBC and the American network CBS, in which he described what he said he experienced while working at the firm between 2010 and 2022.

He was accustomed to finding “anywhere from 50 to 100, 200” defects on fuselages – the main body of the plane – that were due to be shipped to Boeing, he said.

“I was finding a lot of missing fasteners, a lot of bent parts, sometimes even missing parts.”

Boeing declined to comment.

Spirit AeroSystems and Boeing have both come under intense scrutiny after an unused door came off a brand new 737 Max shortly after take-off in January, leaving a gaping hole in the side of the plane. According to investigators, the door had originally been fitted by Spirit, but had subsequently been removed by Boeing technicians to rectify faulty riveting.

The incident prompted the US regulator, the Federal Aviation Administration, to launch an audit of production practices at both firms. It found multiple instances where the companies failed to comply with manufacturing control practices.

Mr Paredes told the BBC that some of the defects he identified while at Spirit were minor – but others were more serious.

He also claimed he was put under pressure to be less rigorous.

“They always made a fuss about why I was finding it, why I was looking at it,” he said.

“They just wanted the product shipped out. They weren’t focused on the consequences of shipping bad fuselages. They were just focused on meeting the quotas, meeting the schedule, meeting the budget… If the numbers looked good, the state of the fuselages didn’t really matter,” he alleged.

Many of Mr Paredes’ alleged experiences at Spirit form part of his testimony in legal action that disgruntled shareholders have brought against the firm.

However, in legal documents he is referred to simply as “Former Employee 1”. This is the first time Mr Paredes, a former Air Force technician, has spoken publicly.

Before his departure from the firm, Mr Paredes led a team of inspectors based at the end of the 737 Max production line.

A second former quality auditor, Josh Dean, whose claims were also to form part of the lawsuit, passed away last week after contracting a serious bacterial infection.

The lawsuit accuses the company of deliberately attempting to cover up serious and widespread quality failings, and exposing shareholders to financial losses when those failings became exposed. Spirit said it “strongly disagrees” with the assertions in the legal action.

Spirit was once part of Boeing and remains the planemaker’s primary supplier. It builds the fuselage for every 737 Max at its factory in Wichita, Kansas, before shipping them to Boeing’s own facility in Renton, near Seattle, Washington. It also makes large parts of the 787 Dreamliner.

It is now in a difficult position. It is haemorrhaging cash, and lost $617m (£494m) in the first three months of the year.

Boeing has agreed to provide financial support, and is in talks to buy back its former subsidiary.

Sources within the aerospace giant insist that efforts are under way to address quality concerns at Spirit, and these have succeeded in reducing the number of faults in parts leaving the Wichita factory by around 80%.

Mr Paredes said both companies were aware of the scale of the problem with defects, and that it was discussed at weekly meetings between quality inspectors from both firms.

Matters came to a head for Mr Paredes personally, he claimed, when he was ordered by his manager to change the way in which defects were reported, in order to reduce their overall number.

After he protested, he said, he was demoted and removed to another part of the factory.

“I felt I was being threatened, and I felt I was being retaliated against for raising concerns,” he said.

Mr Paredes subsequently filed an “ethics complaint” with the company’s human resources department, and wrote to Spirit’s then chief executive, Tom Gentile.

In that email, he said “I have lost faith on the quality organisation here at Spirit and this is my last cry for help”.

Mr Paredes was subsequently reinstated in his leadership role and given back-pay after his complaint was partially upheld. He left the company soon afterwards.

He now maintains he would be reluctant to fly on a 737 Max, in case it still carried flaws that originated in the Wichita factory.

“I’d never met a lot of people who were scared of flying until I worked at Spirit,” he said.

And then, being at Spirit, I met a lot of people who were afraid of flying – because they saw how they were building the fuselages.”