A pilot of a passenger plane says he fell asleep while mid-air - and his co-pilot was asleep at the same time.
The revelation came after a study was commissioned by the British Airline Pilots Association (BALPA), which asked pilots about their working hours.
It found that one in five pilots felt that their ability to fly an aeroplane was 'compromised' more than once a week.
The union says the sleeping incident highlights the growing problem of fatigue among pilots and flight crew, and has warned that this is putting lives at risk.
The pilot, who was in charge of the long-haul flight, said he dropped off for around ten minutes while his over-tired co-pilot took a nap during a scheduled break.
He said the aircraft was on autopilot but admitted it would have been easy to sleep through a warning alarm if anything had gone wrong.
'When I woke up, it was a big adrenalin rush,' he told the BBC.
'The first thing you do obviously is check your height and your speeds and all of your instrumentation.
'The worst scenario is that the autopilot would disconnect itself and then the aircraft would lose or gain height and that would be extremely dangerous as you'd go into the path of oncoming aircraft.
'Now there are warning systems that tell you are deviating from the correct altitude but they are not excessively loud. It would be easy enough to sleep through that, and I probably don't need to tell you what the consequences are.'
The name of the pilot and the airline involved has not been disclosed.
The study found that 45 per cent of 492 pilots questioned had suffered from 'significant fatigue'.
EU proposals to raise flying hours from 900 to 1000-a-year could lead to disaster due to tiredness, says BALPA.
General secretary Jim McAuslan said: 'Fatigue among pilots is a real worldwide problem.
'Tiredness is now accounting for between 15 per cent and 20 per cent of accidents. Now, incredibly, the EU wants to increase flying hours.
'Fatigue among British pilots is growing, as this study shows, and as our members know. UK pilots have also been giving personal testimonies about their own experience of fatigue and what it feels like to be pushed to the limits.
'These are enough to make the hairs stand up on the back of your neck, even under current fatigue regulations. To force them to fly still more hours is, frankly, reckless.'
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