Ryanair’s website will be down for 12 hours for a “system upgrade”, the airline has said.

Customers will be unable to access the site from 17:00 on Wednesday 7 November until 5:00 on Thursday 8 November, meaning they won’t be able to check-in for flights online.

Europe’s biggest airline is advising travelers with flights on Wednesday and Thursday to check-in on Tuesday 6 November; passengers should have already been contacted via email and text.

The Ryanair app will also be down, with customers unable to make new bookings or change existing bookings during this time.

Up to 500,000 customers, either potential or actual, could be affected – given that Europe’s biggest budget airline normally takes some 15,000 bookings an hour and that 350,000 are already set to fly on Wednesday and Thursday.

The budget airline usually charges £55 per person to check-in at the airport and £20 to reissue a boarding pass. These fees will still apply during the website closure, meaning passengers who forget to check-in online before the 5pm cut-off on Wednesday will be subject to a £55 penalty, while those who check-in but fail to download their boarding pass before the site closure will have to cough up £20.

Travelers who are in the middle of trips and may be offline could find out only when they come to check in that the service is not available.

Checking changes to existing flights will also be impossible during the system shutdown.

A Ryanair spokesperson said: “Our existing boarding card reissue fee will apply, which is why we’re giving customers traveling on Wednesday and Thursday sufficient notice to check-in for their flights before the website closure.”

Ryanair’s Robin Kiely said: “As part of our continued digital development, a web closure and system upgrade will take place for 12 hours, from 17.00 (UK time) on Wednesday 7 November, until 5.00 (UK time) on Thursday 8 November, during which time our online check-in service will not be available.

“Flight bookings and changes will also be unavailable during this period. Customers traveling on Wednesday or Thursday should check-in for their flight on Tuesday 6 November.

“We sincerely apologize for any inconvenience caused by this upgrade, which will further improve Ryanair.com, Europe’s largest travel website.”