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.
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."