New bookings and changes to existing flights will also not be possible when the Dublin-based carrier’s website and mobile app are taken offline from 5pm on Wednesday until about 5am on Thursday.
The airline said it has sent emails and text messages to all customers flying on Wednesday or Thursday to advise them to check in online on Tuesday.
Ryanair’s policy is to charge passengers an extra £55 if they need to check in at an airport.
The firm’s online check-in facility is only open between 48 hours and two hours before a flight for passengers who have not paid extra to select a seat.
Ryanair’s head of communications Robin Kiely said the temporary suspension of its website and mobile app was part of the airline’s “continued digital development”.
He went on: “We sincerely apologise for any inconvenience caused by this upgrade, which will further improve Ryanair.com, Europe’s largest travel website.”