An engineer at Boeing has alleged that the aircraft company, in rushing to produce as many planes as possible, is taking manufacturing shortcuts that could lead to jetliners breaking apart.

“They are putting out defective airplanes,” the engineer, Sam Salehpour, told members of a US Senate subcommittee.

Mr Salehpour was giving evidence about Boeing’s 787 Dreamliner, hundreds of which are in use by airlines, mostly on international routes.

He spoke while another Senate committee held a separate hearing on the safety culture at Boeing.

The dual hearings were a sign of the intense pressure on Boeing since a door-plug panel blew off a 737 Max jetliner during an Alaska Airlines flight in January.

The company is under multiple investigations, and the FBI has told passengers from the flight that they might be victims of a crime.

Regulators limited Boeing’s rate of aircraft production, and even minor incidents involving its planes attract news coverage.

Mr Salehpour alleged that workers at a Boeing factory used excessive force to jam together sections of fuselage on the Dreamliner.

The extra force could compromise the carbon-composite material used for the plane’s frame, he claimed.

The engineer said he studied Boeing’s own data and concluded “that the company is taking manufacturing shortcuts on the 787 programme that could significantly reduce the airplane’s safety and the life cycle”.

Mr Salehpour said that when he raised concern about the matter, his boss asked whether he was “in or out” – part of the team, or not.

“’Are you going to just shut up?’ … that’s how I interpreted it,” he said.

Boeing said retaliation is strictly prohibited.

A spokesperson said the company encourages employees to speak up, and that since January it has seen more than a 500% increase in employee reports on a company portal.

The hearing of the investigations subcommittee marked the first time Mr Salehpour has described his concern about the 787 and another plane, the Boeing 777, in public.

Senators said they were shocked and appalled by the information.

Democrats and Republicans alike expressed their dismay with the American aircraft manufacturer.

The company says claims about the Dreamliner’s structural integrity are false.

Two Boeing engineering executives said this week that in both design testing and inspections of planes – some of them 12 years old – there were no findings of fatigue or cracking in the composite panels.

They suggested that the material, formed from carbon fibres and resin, is nearly impervious to fatigue, which is a constant worry with conventional aluminium fuselages.

The Boeing officials also dismissed another of Mr Salehpour’s allegations: that he saw factory workers jumping on sections of fuselage on another one of Boeing’s largest passenger planes, the 777, to make them align.

Separately on Wednesday, the Senate Commerce Committee heard evidence from members of an expert panel that found serious flaws in Boeing’s safety culture.

One of the panel members, MIT aeronautics lecturer Javier de Luis, said employees hear Boeing leadership talk about safety, but workers feel pressure to push planes through the factory as fast as they can.

In talking to Boeing workers, Mr de Luis said he heard “there was a very real fear of payback and retribution if you held your ground”.

The dual hearings added to criticism that has been heaped on Boeing since the door plug blew off an Alaska Airlines Boeing 737 Max as it flew over Oregon.

Major safety failures have pushed Boeing into a crisis that has already resulted in a management shake-up, including the chief executive’s decision to step down at the end of this year.

Senator Ted Cruz (Republican-Texas) said the public is looking to Washington to assure that boarding a plane is not getting more dangerous.

“Flying commercial remains the safest way to travel, but understandably, recent incidents have left the flying public worried. The perception is things are getting worse,” he said.

The Federal Aviation Administration (FAA), which regulates airlines and aircraft manufacturers, was also heavily criticised during Wednesday’s hearings.

The FAA was battered for the way it approved the 737 Max nearly a decade ago without fully understanding a key flight-control system.

Two Max jets crashed in 2018 and 2019, killing 346 people.

Critics continue to accuse the agency of being too cosy with Boeing.

“The FAA needs to be a regulator. They need to do their job. That’s the missing piece right now,” Joe Jacobsen, a former Boeing and FAA engineer, told the investigations subcommittee.

The FAA is now under a new administrator, Mike Whitaker, who has taken a tougher approach to Boeing.

He limited Boeing’s production of 737 Max jets and gave the company until May 28 to produce a detailed plan for how it will fix manufacturing problems and resolve safety concerns.

Boeing is facing separate investigations by the FAA, the Justice Department and the National Transportation Safety Board.

The Justice Department could reopen a 2021 agreement in which Boeing avoided criminal prosecution on a charge of misleading regulators about the Max.

In exchange, the company agreed to pay 2.5 billion dollars (£2 billion) – mostly to airline customers.

All the attention is taking a chunk from Boeing stock, which has tumbled in price by nearly one-third since the Alaska Airlines panel blowout.

Shares of the Arlington, Virginia, company have lost 32% – more than 47 billion dollars (£37.7 billion) in market value.

Senator Richard Blumenthal, the Connecticut Democrat who chairs the investigations subcommittee, and the panel’s senior Republican, Senator Ron Johnson of Wisconsin, have asked Boeing and the FAA for troves of documents going back six years.

Mr Blumenthal said his subcommittee plans to hold more hearings on Boeing and hopes to hear from chief executive David Calhoun.

In interviews and messages to employees, Mr Calhoun has said many times that Boeing is taking steps to improve its manufacturing quality and safety culture.

He called the Alaska Airlines accident a “watershed moment” from which a better Boeing will emerge.

There is plenty of scepticism about comments like that.

“We need to look at what Boeing does, not just what it says it’s doing,” said Senator Tammy Duckworth (Democrat-Illinois).