Concerns about pilot fatigue grew today after it emerged that two cockpit crew members fell asleep at the controls of a long-haul flight.
One of the pilots told the BBC he fell asleep for about 10 minutes while his over-tired co-pilot took a nap during a scheduled break.
The disclosure came after a study commissioned by pilots' union Balpa showed that 45% of 492 pilots questioned had suffered from "significant fatigue".
Around 20% of the pilots said their ability to fly an aircraft was "compromised" more than once a week.
The pilot who nodded off on a flight said the aircraft had continued on autopilot but that it would have been easy to sleep through a warning alert if anything had gone wrong.
He told the BBC: "When I woke up, it was a big adrenalin rush. 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 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."
Balpa is concerned that fatigue problems could worsen under EU proposals to increase flying hours.
Balpa general secretary Jim McAuslan told the Daily Telegraph: "Fatigue among pilots is a real worldwide problem.
"Tiredness is now accounting for between 15% and 20% of accidents. Now, incredibly, the EU wants to increase flying hours."
The fatigue report was carried out for Balpa by a team from University College London (UCL).
Of the 492 pilots questioned, two-thirds were captains.
As many as 40% of the pilots found themselves having to fly more than the regulation hours at least twice a month to cope with the volume of flights.
Balpa said the Civil Aviation Authority does allow these "discretionary hours" to deal with difficult situations but they are supposed to be a rare, not regular, event.
The report said: "Pilot fatigue is an acknowledged contributory risk factor to aircraft accidents."
The UCL team's recommendations included better scheduling to take pressure off pilots and more transparent fatigue reporting systems.
Mr McAuslan said today: "With increasing competitiveness in the industry, there was always a risk that fatigue levels would increase for pilots. This study shows that the risk is real.
"And our major concern is that, far from regulating to tackle this growing problem, the EU is proposing to scrap UK regulations and replace them with more permissive rules."
He went on: "Pilot fatigue has been causing concern around the world. In the USA the authorities have acted in the last two years to reduce pilot duty hours. Yet the EU is planning to go in the opposite direction.
"The UK's current fatigue rules are not perfect, as the UCL study shows, but they are the result of decades of scientific and medical study as well as pilots' operational experience.
"The European Aviation Safety Agency did sponsor research when drafting its EU rules, but it has then substantially ignored the scientists' recommendations."
Mr McAuslan said Balpa had written to Transport Secretary Philip Hammond "asking that the British Government stand up for scientifically-based, flight-time limitations and UK aviation safety standards, and to press the European Council of Ministers to level up standards rather than joining a race to the bottom".
Mr McAuslan said: "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."