Ryanair will power down its website and mobile app for 12 hours this week to allow for a system upgrade.
Passengers have been warned they will not be able to make bookings or check-in during the downtime.
However, the low-cost carrier has confirmed its £55 fee for airport check-in will apply during the outage.
The airline’s web and app services will be offline from 5pm on Wednesday though 5am Thursday.
It has apologised to passengers and urged them to ensure they complete any necessary actions, such as check-in, in advance.
Ryanair operates a 48-hour check-in window before departure for passengers who have not paid for seats or priority boarding.
In a statement, the airline said: "As part of our continued digital development, a web closure and system upgrade will take place for 12 hours from 1700 (UK time) on Wednesday November 7 until 0500 (UK time) on Thursday November 8, 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 November 6.
"We sincerely apologise for any inconvenience caused by this upgrade, which will further improve Ryanair.com, Europe’s largest travel website."
Passengers are advised to check-in in advance and print any necessary documents for travel.
New bookings and changes to flight options and/or passenger information will also be unavailable.
Ryanair has said it has contacted all affected customers by email and text.