Ryanair is shutting its website and mobile app for eight hours today for a systems upgrade.
Online services including flight bookings will be unavailable from 6.30pm tonight until 2.30am Thursday morning.
The low-cost airline urged all passengers travelling today and tomorrow to check-in online before the web closure takes place.
Online check-in will not be available during the maintenance period.
New bookings or changes to existing flights will not be possible during the six-hour closure.
A spokesman said: “This web closure and systems upgrade will take eight hours, during which time our online check-in service will not be available.
“Flight bookings and changes will also be unavailable during this period.
“Customers travelling on Wednesday or Thursday should check-in for their flight on Tuesday.
“We sincerely apologise for any inconvenience caused by this upgrade, which will further improve Ryanair.com, Europe’s largest travel website, as we continue our path to becoming the Amazon of travel.”