Pilots of jet planes have fallen asleep on duty in or over London twice in three years.
A Freedom of Information request to the Civil Aviation Authority (CAA) revealed pilots dozed off in the cockpit in both June 2010 and June 2011.
And the British Airline Pilots Association (Balpa) claims the number of incidents is far higher, happening on a daily basis nationwide.
The CAA insists pilot fatigue is not a safety concern.
The 2010 incident happened on the tarmac in an Airbus A319, when the captain allegedly fell asleep for a short while on turnaround.
It was claimed unrealistic rostering caused flight crew fatigue.
Co-pilot shaken awake
In 2011 the captain of a Boeing 767 was unable to gain re-entry to the flight deck having taken a short break, with the plane airborne over the London area.
He used an emergency code to get in, to find the first officer unconscious. He came round on shaking.
Dr Rob Hunter, head of flight safety at Balpa, said: "A pilot falls asleep on the flight deck on a UK-registered airplane at least once in every 24 hours - and probably a lot more than that.
The evidence the CAA has
is that the fatigue of crew is
not a significant safety concern"
Civil Aviation Authority
"But the regulators and the industry seem to shy away from how serious a problem it is."
Dr Hunter said the large discrepancy between the CAA's figures and their own estimation of the problem was because pilots rarely report themselves falling asleep.
He said doing so risked pilots "writing the evidence for their own prosecution".
There is a possible two-year prison sentence for falling asleep in the cockpit.
Pilots blame fatigue and the number of hours they are asked to work.
But flying conditions may be about to worsen as under a proposed change in European law pilots could be asked to work extra hours more frequently and overnight flying may be extended.
Kathryn Jones, of the CAA, said: "We have no evidence of significant safety concerns with regard to crew and fatigue.
"The evidence the CAA has is that the fatigue of crew is not a significant safety concern."