Ryanair passengers are being warned they will be unable to book flights or check-in online for several hours on Wednesday.
The Irish airline's website and app will be "temporarily unavailable" due to scheduled maintenance from 7pm on Wednesday night.
The online site and app will be out of action "for a number of hours", with no specific end time indicated.
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Passengers who are due to fly on Wednesday evening must check-in online before 7pm.
While the maintenance is ongoing, customers will be unable to make bookings, change flight detail, check-in online or carry out any other actions when it comes to planning a trip.
Ryanair said: "Due to scheduled maintenance, the Ryanair website / app will be temporarily unavailable from 19:00hr GMT on Wednesday, 23rd March for a number of hours.
"Customers who are due to fly must check-in online before Wednesday, 23rd March 19:00hr GMT.
"Customers who wish to make bookings, flight changes / name changes, online check-in and all other aspects regarding a flight booking will be unable to do so during this period of maintenance."
According to its terms of service, Ryanair's check-in policy provides that "all customers can check-in online, free of charge, up to 2 hours before the scheduled flight departure.
Airport check-in is available but charges will apply.
Customers who have not checked in online will encounter an airport check-in fee, which is £/€55 (for flights from Spain €/£ 30) per passenger.
If you have checked in but you have not printed your boarding pass, you will be charged for the boarding pass reissue fee, which is £/€20 per passenger.
This means that people who do not check-in on time for their Wednesday evening flight could be in line for big fees if forced to check-in at the airport instead.
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