Budget airline Ryanair is cancelling up to 50 flights daily for the next six weeks after it "messed up" the planning of pilot holidays.
Kenny Jacobs, chief marketing officer, said the company was "working hard to fix" the problem, after it announced a 2% reduction in scheduled flights until the end of October.
He said: "We apologise to all affected customers for these cancellations. We have messed up in the planning of pilot holidays and we're working hard to fix that."
Ryanair said air traffic control (ATC) delays and strikes, bad weather and a backlog of annual leave to be taken by pilots and cabin crew had led to punctuality falling to below 80% over the last two weeks. 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.
Some customers said last-minute cancellations had left them out-of-pocket due to non-refundable accommodation costs, or with no choice but to book expensive alternative flights or transport. Others said they had been left stranded in their holiday destination and many urged Ryanair to publish a list of all flight cancellations.
One of many customers to write on the airline's Facebook page, Olivia Poole said her flight on Sunday was cancelled "with no explanation".
Ms Poole wrote: "We've shelled out five times our original flight price for last minute, incredibly overpriced flights with another airline as your next available flights were on the day we're due to fly back. I will be going to any length required to receive the compensation as per EU legislation."
Ryanair said a change in the company's holiday year, from April to March to a calendar year from January 1 2018, had seen an increase in holiday allocations as staff used their annual leave before the end of the year.
It said the 40 to 50 flight cancellations each day accounted for less than 2% of its 2,500 daily flights and it would create additional standby aircraft to help restore punctuality.
Mr Jacobs said a "slightly higher number" of flights would be cancelled this weekend.
Robin Kiely, head of communications at Ryanair, said: "We have operated a record schedule and traffic numbers during the peak summer months of July and August but must now allocate annual leave to pilots and cabin crew in September and October while still running the bulk of our summer schedule.
"This increased leave, at a time of ATC capacity delays and strikes, has severely reduced our on-time performance over the past two weeks to under 80%. By cancelling less than 2% of our flying programme over the next six weeks, until our winter schedule starts in early November, we can improve the operational resilience of our schedules and restore punctuality to our annualised target of 90%.
"We apologise sincerely to the small number of customers affected by these cancellations and will be doing our utmost to arrange alternative flights and/or full refunds for them."
Aviation Minister Lord Callanan said: "I am very concerned to see all of these reports of stranded Ryanair passengers. We expect all airlines to fulfil their obligations to their customers and do everything possible to notify them well in advance of any disruption to their journey.
"In the event of any disruption or cancellation airlines must ensure customers are fully compensated and every effort is made to provide alternative travel arrangements."