Ryanair says a number of delays earlier this morning were caused by an IT systems outage.
A number of passengers on twitter say they were unable to check-in online or at various airports.
In a statement, Ryanair has apologised but says the issue has been resolved and flights are back operating.
On Twitter, one passenger said: "Daughter unable to check online last night as website down ..queued for an hour at Gatwick now to pay £55 penalty while staff are struggling with the system down."
Another user said: "My return booking from Nice to Stansted today has disappeared from both app and website."
While another wrote: "Why is your app not working? I have three flights booked but apparently I have none. Why?! I am supposed to be flying home today, this is a joke.
"I need a human, not a bot!"
The carrier has apologised for any inconvenience caused to customers.
