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.
You may also like
'I was at spin class ... ': Doug Emhoff recalls when Kamala Harris told him Biden was dropping out
'No one should go to jail for smoking weed': Harris promises to legalise recreational marijuana
Canada's targeting of envoy Verma 'absurd', says official
'Dying to see my Cholesterol': How Trump responded to Kamala's medical report charge
'Possess, control, impregnate her': Olivia Nuzzi's ex-fiancé claims RFK jr manipulated her
Baba Siddique Murder: Attackers Who Shot NCP Leader Were Suspected To Be Under Influence Of Marijuana; Drug Packet Found At Rented House In Kurla
Brits warned as Spain passes 'royal decree' in holiday crackdown
Lifetime ISA warning as area with the most savers can't even use the funds
Victoria's Secret Fashion Show Returns After 6-Year Hiatus; Gigi Hadid Opens the Runway
Vernon Kay says daughters and wife Tess Daly 'shouted' at him over one-sided issue at home
Over 800 US military veterans released under 'don't ask, don't tell' policy receive honorable discharges
"Matter of serious concern": CPIM on anti-India Khalistani elements operating in Canada
Horror house fire killed mum and 5 children as Doctor dad leaped from window to get help
Omar Abdullah To Take Oath As First CM Of J&K Union Territory Today
Salman Khan Should Give 'Counter Threat' To Lawrence Bishnoi, Says Ram Gopal Varma
Warning to avoid these four popular breakfast foods if you have high blood pressure
Arne Slot 'sent private text' to Virgil van Dijk to influence Liverpool U-turn
Man Utd news: Alex Ferguson's contract terminated as Zinedine Zidane backed for move
Will UAE airlines see one-pilot operation as AI-assisted flying takes off?
'Genius' method to upcycle delivery cardboard boxes into stunning pots for your garden