The website and app will be out of service for eight hours this week, meaning passengers flying could be hit with a £55 airport fee if they fail to check-in online before the outage.
The website is due to go down between 7:00pm on Wednesday 16 October until 3:00am on Thursday 17 October, so passengers have been urged to check in earlier if they are flying this week.
Ryanair announced the news on its website: "Our website / mobile app is undergoing essential scheduled maintenance from 19.00hrs Wednesday, 16 October - 03.00hrs (GMT) Thursday, 17 October.
"Passengers who are due to fly during this period should check-in online before 19.00hrs (GMT) on Wednesday, 16 October. We sincerely apologise for any inconvenience caused as a result of this brief downtime."
Passengers with pre-reserved seats can check in up to 60 days before their outbound and return flight, but those who opted for randomly allocated seats have a shorter window starting 24 hours before the flight until two hours pre-departure.
Those who miss the deadline can still check in at the airport, but they could be charged a £55 admin fee, or £30 for flights from Spain.
It could affect passengers flying from any UK airport serviced by Ryanair, which includes Birmingham, Bournemouth, Bristol, East Midlands, Leeds Bradford, Liverpool, Stansted, Gatwick, Luton, London City, Manchester and Newcastle.
The outage comes just before the half-term holiday rush which is due across the country in under two weeks, from Monday 28 October to Friday 1 November.
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