Plans to change how pilots' working hours are regulated have been rejected by MEPs amid safety fears.
The European Commission wants to standardise the varying time limits for flying across member states.
But pilots' union the British Airline Pilots Association (Balpa) had claimed the plans could mean UK aircrew landing planes after being awake for 22 hours.
MEPs on the European Parliament's transport committee voted to block the new rules by 21 to 13.
Balpa argued the new rules, which could now be decided on in October by the full European Parliament, would allow pilots to:
- land an aircraft after being awake for 22 hours
- fly on the longest-haul flights with only two crew rather than the three at present
- work up to seven early starts in a row rather than the current three allowed
Air travel regulator the Civil Aviation Authority (CAA) had backed the changes, saying they would improve safety for UK passengers flying on some European airlines.
And it dismissed the union's claims that under some circumstances - with standby and flying time combined - pilots could be awake for 22-hours.
"It is important to understand this scenario will almost never happen," a CAA spokesman said.
The European Commission said pilots would have and "must use" sleep periods when on call away from the airport, which would limit the consecutive hours they could be awake to 18.
But a Balpa spokesman said rejection of the new rules reflected "pilots' concerns about the way the rules had been put together without proper scientific scrutiny and underpinning evidence".
General secretary Jim McAuslan described the proposals as "botched".
"The commission must now go back to the drawing board and work with pilots and scientists to develop rules on flying time and tiredness that are based on evidence and expert experience.
"Pilots will continue to urge UK ministers, MEPs and the CAA to protect the safety of our skies and help pilots make every flight a safe flight."
'False and irresponsible'
The European Commission had argued the proposed new regulations would ensure airlines operated the same flying rules across Europe, and flight time limits would be lowered.
"Any impression that is being given that the proposals would weaken those rules is false and irresponsible," a commission spokesman said.
The UK government had also backed the plans.
Ahead of the vote they were described as "balanced and safe" by the Labour chairman of the European Parliament's transport committee.
"We are trying to get a more uniform system in place that the whole of Europe can work to," said MEP Brian Simpson.
"The proposed changes will either continue to ensure high levels of aviation safety or will improve the present standards in many member states."
The vote came after an incident log from last month which was widely reported to suggest that two pilots on an Airbus passenger plane had fallen asleep at the same time while the aircraft was mid-flight on autopilot.
The CAA later clarified that the pilots, while suffering symptoms of severe fatigue, had not fallen asleep at the same time.
Last week, a survey of 500 commercial pilots by the pilots' union BALPA suggested more than half had fallen asleep while in charge of an aircraft.
One in three said they had woken up to find their co-pilot sleeping as well.