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
Bollywood's dream girl lives a luxury life
Jaishankar, Pak PM shake hands but little hope of thaw; Pak says ball in India's court for bilateral
Important policy change for night-time flyers at major UK airport
US says Indian probe team's visit reflects New Delhi's serious intent
Explosion Reported In Gubbängsvägen, Stockholm: What We Know
Thomas Tuchel comments could spell bad news for Trent Alexander-Arnold with England
Baba Siddique Murder Case: Fugitive Shubham Lonkar Was Taken Into Custody In Salman Khan Residence Firing Case
Illinois: Fire Breaks Out In Elgin, Garage Destroyed On St. Charles Street
Roman Kemp forced to halt The One Show as he makes big announcement
Navi Mumbai: Doctors Successfully Remove 6.8 Kg Pumpkin-Sized Adrenal Tumor From 44-Year-Old Woman At Fortis Hiranandani Hospital, Vashi
Eyeing Product Innovation, SaaS Startup Neuron7 Raises $44 Mn From Smith Point Capital
Deepika Padukone returns to work after becoming a mother
Baba Siddique Murder Case: 4th Accused Arrested From Bahraich, Police Say He Funded Shooters
Cong gears up for campaign in Wayanad as workers put billboards of Priyanka Gandhi
Why Asmongold's 'Zackrawrr' Twitch Account Was Banned
Thomas Tuchel England contract details as new manager set for late start date
Ozzy Osbourne's guitarist Jake E. Lee shot several times while on dog-walk in Las Vegas
Immigration and border policy: Where do the 2024 presidential candidates stand?
Farmers warn UK 'walking into food shortages' as they beg Rachel Reeves to reconsider cuts
Five day-to-day things 'highly intelligent people' find exhausting say experts