Both pilots on an Airbus passenger plane were asleep at the same time with the UK-operated 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 sleep in the previous two nights.
Details of last month's incident, on August 13, come at a time when UK pilots' organisation Balpa is unhappy at proposed European changes to flight-time regulations.
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.
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.
– CAA spokesman
Details of the incident, logged by the CAA as a mandatory occurrence report, were obtained by a news agency which 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.