Ryanair passengers are reporting problems with the process of refunds or compensation for cancelled flights, saying information supplied by the airline is incorrect and calls are going unanswered.
Angry customers are inundating Ryanair's Facebook page with reports of problems with the airline's refund and compensation process, and its complaints tracker.
Many are also complaining their calls and messages through the airline's call centre and online "chat" service are going unanswered or being cut off.
Holly Jayne Austin posted: "Has anyone managed to get through to Ryanair or had a refund? Won't let me refund and they are just cancelling my call" while Laure Savoy wrote: "How can I ask for the EU261 compensation and refund if on your website the cancelled fight number is unknown for you?"
Lena Heritage wrote: "Please let me know how we can contact you! You emailed us two days ago to notify us that our flight home from Barcelona has been cancelled, instructing us to sign in to our account to rearrange an alternative flight.
"We have since been stuck in a constant loop of being asked to verify our account and being asked to change our password.
"We've not been able to get through on the extortionately-priced phone line (repeatedly in a queue then eventually cut off)."
Birinder Singh Tiwana posted: "I applied for refund and compensation by filling a form on their website and got a customer reference number of seven digits via email which does not work on their complaint tracker system.
"I tried to chat to their agents about this but the agent had no answer and disconnected the chat instead."
The complaints follow some other Ryanair passengers reporting having to pay a second time to pick seats or take hold luggage when rebooking cancelled flights.
The travellers who were offered "free" replacement flights discovered they were not refunded for charges on extras paid on their original booking, leaving them having to pay twice.
The Dublin-based carrier - which is shelving up to 50 flights daily over the next six weeks - said it was aware of the issue and any customers who were double-charged for seats or hold luggage will be refunded.
Ryanair faces a compensation bill of up to 20 million euro (£17.7 million) over the cancellations brought about by the over-allocation of pilots' holidays during a relatively busy period.
The airline said it was cancelling flights at airports where it ran the busiest schedules so it would be easier to place passengers on alternative flights.
Customers affected by cancellations which run until October 28 will be emailed offers of alternatives or full refunds and details of their compensation entitlement, the budget airline confirmed.
Michael O'Leary, the airline's chief executive, told a press conference on Tuesday that it was clear the debacle had a "large reputational impact" and apologised.
Ryanair said a ll 315,000 customers affected by the cancellations received an email on Monday advising them of their flight changes and offering alternative flights, refunds and EU261 notices.
The airline said it expected to have re-accommodated more than 175,000 customers on other Ryanair flights and to have processed refunds for more than 20% of those affected by close of business on Wednesday.
Ryanair's Kenny Jacobs said: " We apologise sincerely to each and every one of the 315,000 customers whose original flights were cancelled over a six week period in September and October, while we work to resolve this short term rostering failure.
"We have taken on extra customer service teams to speed up the rate at which we accommodate and action alternative flight requests or refund applications. We expect to have the vast majority of these completed by the end of this week.
"The vast majority of these requests are being dealt with online, but as our call centres and chat lines are extremely busy we ask affected customers to bear with us as we do everything we can to respond to their requests and try to resolve any problems we have created for them, for which we again sincerely apologise."
Which? managing director of home products and services, Alex Neill, said: " Ryanair's approach to informing affected passengers about compensation falls woefully short. It is legally required to spell out compensation rules when a flight is cancelled and, in our view, have so far failed to do that, leaving passengers hunting around for information.
"This is another blow for the thousands of passengers who have already had to endure huge inconvenience as a result of this fiasco. The airline must now automatically compensate eligible passengers without them having to go through the additional hassle of making a claim."