Ryanair passengers were stranded in Greece after their UK-bound flight left without them as airports grapple with passport control delays.
The flight from Athens International Airport to London Luton on Sunday reportedly left without 20 to 50 passengers, some of whom became visibly angry, prompting airport authorities to step in, the BBC has been told.
It is the latest incident since the EU's Entry Exit System (EES) was brought in to track when non-EU citizens enter or leave the Schengen free movement area.
Ryanair blamed border delays, while the airport said it had been experiencing congestion linked to "additional processing requirements", though neither directly said EES was responsible.
In April, Greek tourism minister Olga Kefalogianni promised British passengers would not face biometric checks or be "burdened" by bureaucracy when travelling to Greece this summer.
But the situation is unclear after the Greek Foreign Ministry later disputed that any exemption existed.
Milo Boyd, a Daily Mirror travel writer who managed to board Sunday's Ryanair flight to Luton, said he had not had his fingerprints scanned or his photo taken on entry or exit at Athens.
He told the BBC there had been a "mega queue" of several hundred people at both security and passport control amid stifling temperatures.
He and his wife made it through 10 minutes before the gate closed, but he said at least 20 others had arrived too late.
"These poor people were pleading with the Ryanair staff to let them through - one guy was crying, another guy looked like he was about to explode," Boyd said.
Boyd said people stuck on the other side of the gate were shouting "you can't do this" and "this is your stupid airport's fault, how can you not let us on?"
As frustration grew, airport authorities stepped in "to maintain orderly operations" after "a number of passengers expressed their dissatisfaction", the airport told the BBC.
A spokesperson said there had been "periods of congestion at passport control in the departures area due to high passenger volumes and the additional processing requirements associated with travel to non-Schengen destinations".
They added: "As is currently the case at many European airports, passenger flows on certain routes may experience increased processing times as new border-control procedures continue to be implemented and refined."
Ian Collins tweeted Ryanair to say it was "utterly disgraceful you left my daughter (and half your passengers) at the gate in Athens today".
Ryanair said in a statement that "a number of passengers" did not board in time "due to delays caused by border control at Athens airport".
It added: "All passengers that were at the boarding gate when this flight from Athens to London Luton boarded, travelled without incident."
The flight is understood to have missed its air traffic control slot and departed an hour late after the missing passengers' bags were unloaded.
Other passengers have reported similar delays, including Karen Alarcon Cabada from California.
She told the BBC she had been stuck in an hour-long queue at Athens airport on Monday when it had taken "forever to check one's passport".
In April, about 100 Manchester-bound Easyjet passengers were left stranded in Milan after queues of up to three hours at passport control.
In May, the UK boss of budget airline Wizz Air told the BBC that British holidaymakers should now arrive at European airports three hours in advance.
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