A study of 6,000 pilots says that between 43% and 54% had nodded off "involuntarily" while flying. And one third of these claimed they had woken up to find their colleagues were also asleep.
The results of the ECA study come at a critical time, because last month, the European Aviation Safety Agency published proposals for new rules on flight and duty times and rest requirements for pilots.
Under the new rules, pilots could be expected to land commercial jets after being awake for stretches of up to 22 hours.
The proposals have been criticised by pilot organisations including the ECA, which says they will cause "significant consequences" for passengers.
The ECA is campaigning to highlight the safety issues caused by long flying hours. It says that fatigue among pilots is a common, dangerous and under-reported phenomenon in Europe.
Speaking to the Daily Express, a spokesman for the pilots' group said: "Long duty and standby hours, night flights and disruptive schedules often result in long times awake, sleep deprivation and are followed by insufficient rest and poor sleep opportunities."
In October this year a Freedom of Information request by The Sun revealed that two pilots had fallen asleep while in charge of passenger planes. In both incidents, captains who had been on breaks had returned to the cockpit to find their co-pilots slumped over the controls.
"Fatigue impairs the judgment and ability of air crews to react quickly - with potentially disastrous consequences, as demonstrated by recent accidents," said Philip von Schoppenthau, ECA general secretary.
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