Ryanair passengers were left unable to check in ahead of their flights due to a "systems outage" which caused delays at airports across Europe.
Twitter users complained of "chaos" at Stansted Airport on Saturday morning, with many struggling to find their booking details online.
The budget airline confirmed that the outage resulted in disruption to airport check-ins and boarding.
Meanwhile, photographs posted on social media from Rome Ciampino airport in Italy show large queues forming at the Ryanair check-in desks.
Romana McCallum wrote: "Why is a flight allowed to leave when the boarding system crashes mid-boarding, leaving passengers at the terminal?
"Only 3/4 of our party made it through and the remaining 1/4 are stuck at Stansted. Utterly irresponsible, expensive and huge waste of time: Why?"
Vicky Graham-Stevenson complained to Ryanair on Twitter .
She said: "can you please tell me why my daughter has just been charged £55 to check in at the airport when she hasn’t been able to check in online due to the website keep kicking her out every time she tried".
One passenger on a Stansted to Faro flight said: "Basically we’ve been on the tarmac for 90 minutes and they say it’s a country wide system failure.
"I've seen people on twitter saying they’re stranded".
A spokesman for Ryanair said: "This morning (September 14) we suffered a system outage which caused some short delays at airport check-in and boarding.
"The system has been restored and flights are operating as normal. We apologise for any inconvenience caused."
Many of the people who posted on Twitter complained they could not find their flight bookings on the budget airline's mobile app or website.
Ryanair states on its website that people who do not check-in online will be charged £55 per passenger to do so at the airport counter.
A spokesman for Stansted Airport said: "There was a short disruption to some of Ryanair's systems earlier today at Stansted.
"No other airlines have reported problems this morning."
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