Ryanair is being urged to release a full list of the flights it is cancelling because it "messed up" the planning of pilots' holidays.
The budget airline is cancelling up to 50 flights per day for the next six weeks but has only published details up to Wednesday.
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.
Aviation minister Lord Callanan said he is "very concerned" by the Dublin-based carrier's cancellations.
He went on: "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."
Rory Boland, travel editor of consumer group Which?, said the airline must arrange alternative flights or provide a full refund.
He added: "It's also essential that Ryanair release a full list of flights that will be affected so that passengers have as much time as possible to make alternate arrangements."
Ryanair chief marketing officer Kenny Jacobs said the company will "continue to send regular updates".
He went on: "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 below 80% over the last fortnight.
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.
Writing on Ryanair's Facebook page, Lucy Dwyer said she is due to travel on Thursday and needs to know if the flight will go ahead.
She wrote: "It's three days before the flight. Can you tell me before I book airport transfers? This is an absolute joke. My anxiety is through the roof! Thanks Ryanair."
Another passenger, Lizzie Gayton, said she had to spend almost £700 to get home on Sunday after her flight from Lisbon to London Stansted was cancelled with less than 48 hours' notice.
"You weren't able to offer me another flight in time for me to get back for work," she wrote.
"It is lucky that I had a credit card on me and don't have six kids to pay for! It is outrageous to think you are treating paying customers like this."
Harriet Kathryn Ross wrote: "What they need to do is confirm and publish a schedule of which flights will be cancelled over the next six weeks... it's wrong to leave people in suspense at the last minute. It's not fair."
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.