A spokesman said this figure was "unacceptable" and the company has apologised to affected customers, who it said will be offered alternative flights or refunds.
Irish LCC Ryanair estimates that crew-related cancellations over the next six weeks will cost around €25 million ($29.9 million), with CEO Michael O'Leary describing the situation as "a mess of our own making".
RTE News reports Ryanair's chief operations officer Michael Hickey has asked pilots to work for between one and two blocks of five days of leave during September and October. The issue will not recur in 2018 as Ryanair goes back onto a 12-month calendar leave year from January 1 to December 31.
WHEN Ryanair convinced many of its pilots to take fewer holidays during peak summer-travel season, it probably thought it was being clever.
Speaking to Ryanair customer service, they have been told to keep all of their receipts in the hope that they can claim compensation.
Also grounded are two Edinburgh-Stansted return flights on Friday, one return flight between Glasgow and Carcassonne in southern France on Saturday, and one return Edinburgh-Brussels flight on Sunday.
The letter shows Ryanair pilots were only informed on 13 September of the staff shortage facing the company yet Mr Hickey outlined that it knew past year they may face a leave backlog. I apologise sincerely to all our customers for any worry or concern this has caused them over the past weekend.
Budget airline, Norwegian airlines have poached 140 pilots from troubled Ryanair and has recruited 400 pilots since the start of this year.
The Irish airline said it was preparing for up to 20 million euros ($23.9 million) in compensation claims and 5 million euros in lost fares as a result of the cancellations, although analysts estimated the total cost could be higher.
Ryanair says that in most cases it will offer you an alternative flight.
IALPA says more than 700 pilots have left Ryanair this year. "We clearly messed up in our rostering department", O'Leary said yesterday.