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 5pm on Wednesday 7 November until 5am on Thursday 8 November, meaning they won’t be able to check-in for flights online.

Europe’s biggest airline is advising travellers 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.

The closure could affect passengers who do not pay to select seats, and have calculated that the later they check in, the better the seat they are assigned. Free online check-in opens 48 hours before departure.

Travellers 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 travelling 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 travelling on Wednesday or Thursday should check-in for their flight on Tuesday 6 November.

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

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Ryanair hit headlines earlier this month when it reduced its free hand luggage allowance from two bags to one.

Passengers who don’t pay from £6 for priority boarding or from £8 to check a 10kg bag can now only take one mid-size bag on board.

However, the airline has said there will be a “grace period” until the end of November, during which gate agents have been told to take a “reasonable approach”.

Ryanair told The Independent in a statement: “While our new bag policy came in to effect on 1 November, we have briefed our gate agents to take a reasonable approach while customers get used to the new rules over the month of November.”