Engineers building Boeing’s 787 Dreamliner aircraft have serious concerns about its safety, according to a new documentary.

A leaked memo also reveals that suppliers building crucial parts of the aircraft were given permission to deviate from normal manufacturing procedures amid pressure to hit a tight schedule.

The revelations threaten to reopen concerns about the Dreamliner, once branded the ‘aircraft of the future’ by President Barack Obama, but temporarily grounded in 2013 after a series of battery fires.

'Aircraft of the future': The Dreamliner was temporarily grounded in 2013 after a series of battery fires

The allegations, disputed by Boeing, are contained in a documentary by the investigative unit of Qatari-owned news channel Al Jazeera and shown to the Daily Mail.

Undercover footage filmed by a Boeing employee at the firm’s Charleston, South Carolina, assembly plant shows engineers casting doubt on the quality of workmanship there.

One member of staff says: ‘I wouldn’t fly on one of these planes,’ citing the ‘quality’ of work done at the plant.

Of 15 employees asked if they would fly on the Dreamliner in the undercover film, ten said that they would not.

In one piece of footage, a machinist compares the plant’s work unfavourably to Boeing’s Everett plant in Washington.

He complains that ‘everybody here is being pushed to meet the f*cking schedule regardless of quality.’

Boeing said that while it has been confronted with the allegations, it could not comment on the undercover film because it is yet to see it.

It said: ‘Safety is, and always has been, our priority.’

‘We are committed to working with our customers, the U.S. Federal Aviation Administration, other regulatory agencies and stakeholders to ensure the integrity of our production system.’

Slick: The planes offer greater comfort and improved energy efficiency

It added: ‘We also must question the motives of those making anonymous allegations.’

‘It appears their intent is to harm the company and not to further the cause of safety.’

But allegations about the South Carolina plant were made by people who were unaware they were being filmed.

The airline added: ‘At Boeing we take pride in promoting an open culture in which senior management supports employees who bring forward tough issues to the attention of management and colleagues, and there are channels both within Boeing and with safety regulators in which legitimate concerns can be raised.’

Engineers were also filmed saying they feared colleagues working on the aeroplane were using drugs.

One machinist is filmed saying that ‘coke and painkillers’ could be obtained at the plant and that people would go out at lunch to ‘smoke one up’, referring to a cannabis cigarette.

The employee who filmed the footage said: ‘As far as I know, random drug tests just don’t happen’.

Boeing said: ‘Drug testing of employees is done in accordance with Boeing policy and procedures across all facilities in accordance with applicable laws.’

‘Boeing thoroughly investigates any employee reports of policy deviation, and appropriate corrective action is taken if needed.’

The makers of the film, called Broken Dreams: The Boeing 787, also obtained a memo sent by Boeing to suppliers making parts of the aircraft such as the wings and fuselage.

Concerns: One member of staff was filmed saying: ‘I wouldn’t fly on one of these planes'

With delays and work stoppages threatening the Dreamliner’s release date, Boeing told suppliers in 2010 that ‘schedule may require deviations to the preferred process’.

Shown the memo during the course of the documentary, 32-year Boeing veteran Cynthia Cole, former head of the engineers’ union, was stunned.

She said: ‘You don’t change your quality process for schedule. I don’t see how these people who write these things and agree to these things… how they sleep at night.’

‘As an engineer I find that reprehensible. I’m not flying on the 787. I’ve been avoiding flying on a 787 and seeing this, I’d definitely avoid flying on a 787.’

Boeing said: ‘The process specifications referenced in this document from 2010 were fully consistent with Boeing’s robust quality assurance system.’

‘While we will not discuss in detail our proprietary production processes, we noted that the document itself concludes by saying that the process changes “do not signify authorisation to ship or accept parts which do not meet engineering and quality requirements.”’

It added that production at factories where work had stopped was only allowed to continue with Boeing ‘monitoring for quality’.

The Dreamliner took off for its maiden flight in 2009, boasting an array of futuristic features to please passengers, as well as the latest technical specifications.

Investigation: Al Jazeera's Investigative Unit finds some workers with quality concerns

The twin-engine jet, which typically seats 234 people, is 20 per cent more fuel efficient than the generation of aircraft it replaces and can fly for 10,000 miles.

This is largely because it is made with 50 per cent ‘composite materials’ including plastics and carbon fibre, rather than with aluminium.

Windows are 30 per cent larger, affording passengers a better view, while the amount of light from outside can be modified at the touch of a button, rather than pulling down a shade.

The Dreamliner also operates at a lower cabin pressure and uses LED lighting, both of which help reduce jetlag and tiredness.

But its development and worldwide launch was among the most difficult experienced by a commercial airliner.

It was originally due to enter service in 2008 but repeated setbacks delayed its first commercial flight until 2011 and sent its cost spiralling to an estimated $32billion.

And the entire global fleet was briefly grounded in 2013 after two fires caused by its powerful lithium-ion batteries.

The aircraft’s problems do not appear to have deterred major airlines, which have ordered more than 1,000 of the planes.

It is favoured by long haul carriers due to its ability to fly long distances, its fuel efficiency and the enhanced comfort for passengers.

Former flag carrier British Airways has 42 on order, while Sir Richard Branson’s Virgin Atlantic has ordered 15, with options to buy up to 28 more.