Hundreds of Ryanair flights will not take off as planned on Friday due to pilot strikes in five countries.
Staff in Ireland, Germany, Sweden, Belgium and the Netherlands are staging a 24-hour walkout over pay and conditions.
The airline said 396 flights have been cancelled as a result, forcing passengers who planned to travel on Friday to rebook or take different routes.
Ryanair planes have a capacity of 189, meaning more than 74,000 passengers could be affected.
The Irish budget airline said the strikes were "regrettable and unjustified" and called for unions to come back to the negotiating table.
Despite the walkouts, 85% of its scheduled flights, more than 2,000, will operate as normal, Ryanair said.
"Ryanair took every step to minimise the disruption and we notified our customers as early as possible advising them of their free move, refund or reroute options," the carrier said.
"The majority of customers have already been accommodated on another Ryanair flight.
"We want to again apologise to customers affected by this unnecessary disruption and we ask the striking unions to continue negotiations instead of calling any more unjustified strikes."
One customer described the airline as a "headache", complaining they had had difficulty getting a quick response after contacting the firm on their live chat.
They tweeted: "#ryanair cancelled my flight in the last min because of pilots strike. They offer me to change my tickets online which it's not possible because of their system crash. Noone is on the phone and livechat. They even don't do a refund. Ryanair is an headache."
Another customer said she would miss work meetings and a doctor's appointment due to a cancelled flight.
She wrote: "Many thanks to @Ryanair for cancelling my flight home + ensuring all of the de-stressing I have done on this trip is cancelled out in an instant."
In June Ryanair signed an agreement with the Unite union, giving hundreds of cabin crew employees full consultation rights and collective bargaining.
The airline said at the time that it was "a further sign of the progress Ryanair is making with trade unions since our December 2017 decision to recognise them".
The Unite agreement came six months after the airline signed what was described as an "historic" recognition deal with the trade union representing pilots.
Under the agreement, the British Airline Pilots' Association (Balpa) was recognised as the sole trade union representing all of Ryanair's 600 employed pilots based in the UK.