But the company has not given details of all the planes being grounded.
And passengers are demanding to know if their travel plans can go ahead.
Ryanair is pulling the plug on up to 50 flights a day until the end of October.
It has been forced to take action after problems with its holiday rota for pilots.
Up to 350,000 customers are likely to be affected by the airline's decision to cancel flights
Up to 350,000 passengers could see their flights cancelled.
Grandmother Karen Litton, who is due to fly from Knock in Ireland to Stansted on Wednesday, said: "It is very frustrating. Everything seems to have just physically shut down.
"It is just disgraceful that Ryanair won't tell people if their flights have been cancelled.
"A full list should be made available immediately so that people can make alternative plans" she said.
But a travel expert says the airline will have to put customers on a rival line if a suitable alternative is not available
Ryanair's marketing officer Kenny Jacobs said: "We have messed up in the planning of pilot holidays and we're working hard to fix that."
Most of the cancellations are due to a backlog of staff leave, which has seen large numbers of the airline's staff book holidays towards the end of the year.
Customers do have rights under the European Passenger Rights legislation.
Travel expert Simon Calder said: "The rules say if the airline doesn't have a suitable alternative flight, you have to be booked on a rival airline."
And he said passengers should be able to claim compensation for the cancellations.