Pilots' union Balpa raised fears of future crashes after revelations of a potentially deadly incident where both pilot and co-pilot fell asleep at the controls of their passenger plane.

One of the pilots reported to the Civil Aviation Authority (CAA) that they had both nodded off at the same time leaving the British-operated Airbus 330 on autopilot.

Each had only five hours' sleep over the previous two nights before the incident on August 13.

Balpa said the revelation "comes as no surprise" and added it had "repeatedly warned the CAA of the risk of both pilots falling asleep."

It revealed that a survey of members found more than half of pilots had fallen asleep on the flight deck.

Of the 56 per cent who admitted nodding off, as many as 29 per cent said they had woken up to find the other pilot asleep.

The union warned that impending EU changes to flight safety rules mean August's incident will not be the last.

"The CAA has been far too complacent about the levels of tiredness among British pilots," said Balpa general secretary Jim McAuslan.

"The CAA and government are backing EU cuts to UK flight safety that will increase tiredness among pilots and the risk of dangerous incidents."

Of the 500 commercial pilots polled 31 per cent did not believe their airline's culture lent itself to reporting tiredness concerns.

Unprompted, 49 per cent said pilot tiredness was the biggest threat to flight safety - three times more than any other threat.

On Monday the European Parliament will vote on new rules for pilots' flying hours, which Balpa warns would hit safety standards.

The trade union called on MEPs to back a motion requiring the European Commission to withdraw the proposals and subject them to proper scientific scrutiny.

It said the flaws in the proposals include allowing pilots to land an aircraft having been awake for 22 hours.

A CAA spokesman said last month's incident was serious but isolated and "I think lessons will be learnt from this."

He added: "We don't know why the pilots had so little sleep."

The CAA did not say which airline was involved nor where the plane was travelling.