The carrier is switching over to a new calendar system
The Irish airline Ryanair has confirmed it is to cancel 40 to 50 flights daily for the next six weeks.
It says this is because of increased holiday allocations to pilots and cabin crew.
The airline is moving the airline's holiday year from an April to March calendar year to a January to December one.
The change is to kick in from January 1st 2018.
In a statement, the carrier says: "Ryanair has operated at record schedule and traffic levels during the peak summer months... but has a backlog of crew leave which must be allocated before December 31st 2017".
It says this has to be done in order to switch to a calendar leave year as required by the IAA.
It also says air traffic control capacity restrictions in the UK, Germany and Spain - as well as French strikes and adverse weather - have given rise to significant delays in recent weeks.
"Ryanair’s on-time performance has declined from 90% to under 80% over the past two weeks, a figure that is unacceptable to Ryanair and its customers", it says.
"By reducing its scheduled flying programme over the next six weeks by less than 2% (of its over 2,500 daily flights), the airline will create additional standby aircraft which will help restore on-time performance to its 90% average."
The airline says it apologises sincerely for the inconvenience caused.
It says affected customers will be contacted directly and offered alternative flights or full refunds.