Concerns have been raised over an incident in which both pilots on a UK-operated Airbus passenger plane were asleep at the same time, with the aircraft flying on autopilot.
One of the pilots indicated in a report to the UK's Civil Aviation Authority (CAA) that the pair nodded off after both had only five hours of sleep in the previous two nights.
Details of the 13 August incident come at a time when UK pilots' organisation Balpa is unhappy at proposed European changes to flight-time regulations.
A CAA spokesman said: "This was a serious incident but an isolated one. I think lessons will be learnt from this. We are circulating this report within the industry."
He added: "We don't know why the pilots had had so little sleep before this flight. They were taking it in turn to have rest periods, with the one always checking the autopilot and it looks as if both fell asleep at the same time."
Details of the incident, logged by the CAA as a mandatory occurrence report, were obtained by a news agency that had asked for incidents of pilot fatigue.
The CAA did not say which airline was involved nor where the aircraft, an Airbus A330, was travelling.
The report was headed: "Flight crew suffering from symptoms of severe fatigue."
It went on: "Reporter [almost certainly the captain] suggests both members of flight crew had only five hours sleep in two nights due to longer duty periods with insufficient opportunity to sleep.
"Both crew rested for 20-minute rotations and fell asleep."
Balpa said: "In the UK we have a strict set of flight safety rules which govern how long and how often a pilot can fly before their performance is impaired.
"The EU is proposing more permissive flight safety rules which would allow pilots to be flying aircraft whilst dangerously fatigued.
"These rules were not developed using scientific data and could have a grave impact on the safety of UK aviation."
Balpa said the EU proposals were "flawed in many areas", with pilots being legally allowed to land an aircraft having been awake for 22 hours, pilots operating longer-haul flights with only two crew rather than the current three, and pilots possibly being forced to work up to seven early starts in a row.
The CAA said: "We understand that Balpa are not happy with the proposals but we think overall it is a good package and not much different to what we have now."