EasyJet has cancelled 200 flights after being hit by IT problems and advised customers to check their booking before going to the airport.
Flights due to depart between midday and 2pm GMT were affected, the low-cost carrier said.
"Our team of IT specialists is working to restore the systems as soon as possible," easyJet said.
The company had earlier said in a statement "flights due to depart between 1pm and 3pm UK time today may be impacted".
It advised customers to use its flight tracker to check the status of their booking.
EasyJet customers on social media have already been posting about cancelled flights from multiple locations, both in the UK and Europe.
Ana Finch tweeted "don't even bother coming to the airport, checked our bags, sent us to a gate, said our flight was boarding, then found hundreds of other passengers all being told the flights are cancelled as we were meant to board".
Callum Lake posted: "@easyjet you've cancelled my flight I'm stuck in Prague, your website isn't working it's not letting me manage my bookings what do you expect me to do?"
Another user, @jxdieb, said her flight had been cancelled two hours before boarding and that she is "stranded in Paris until tomorrow with an autistic child".
EasyJet said: "We advise customers due to travel with us today to continue to check Flight Tracker to check the status of their flight before making their way to the airport.
"We apologise for the inconvenience caused and would like to thank customers for their patience as we work to resolve this as soon as possible."