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.
The airline has cancelled 16 flights either into or out of Stansted airport in Essex on Monday.
Only two flights between Luton and Faro in Portugal are affected on Tuesday.
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.
Rory Boland, travel editor of consumer group Which?, said the airline must arrange alternative flights or provide a full refund.
Aviation minister Lord Callanan said he is "very concerned" by the Dublin-based carrier's cancellations.
The consumer group Which? is calling on Ryanair to allow passengers to be able to cancel future flights without penalty until the full list of cancelled flights is published.
The group also wants Ryanair to make sure people are re-booked on to flights, refunded for reasonable additional expenses due to re-routing and paid the statutory compensation that many passengers will be legally entitled to under the Denied Boarding Regulations.
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 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 past fortnight.
Passenger rights according to Which?
For those passengers who are currently stranded, they will be entitled to statutory compensation (including accommodation, travel etc if they are being re-routed)
Any passengers travelling in under two weeks will be entitled to compensation, unless they have been put on a similar flight (similar time, same day etc) at no extra cost.
If the re-routed flight arrives at its destination within 2 hours of the original flight (for short haul), 3 hours (for medium haul) and 4 hours (for long haul), Ryanair may reduce the compensation by 50%.
Cancelled flights at Stansted and Luton on Tuesday 19 September
Ryanair says that passengers can assume their flight is operating normally unless they receive an email
- FR 2672 Stansted to Rome
- FR 2642 Stansted to Riga
- FR 8868 Stansted to Tours
- FR 372 Stansted to Biarritz
- FR 2373 Stansted to Gdansk
- FR 1905 Stansted to Bari
- FR 2466 Stansted to Szczecin
- FR 5992 Stansted to Madrid
- FR 5991 Madrid to Stansted
- FR 8347 Stansted to Porto
- FR 8348 Porto to Stansted
- FR 9207 Faro to Luton
- FR 9206 Luton to Faro
Cancelled flights at Stansted and Luton on Wednesday 20 September
15 Stansted Ryanair flights are cancelled on Wednesday
- FR 124 Stansted to Ancona
- FR 8592 Stansted to Bergerac
- FR 805 Stansted to Knock
- FR 8445 Stansted to Budapest
- FR 2295 Stansted to Dusseldorf Weeze
- FR 2352 Stansted to Cologne/ Bonn
- FR 1788 Stansted to Dortmund
- FR 32 Stansted to Oslo Torp
- FR 7973 Stansted to Basel
- FR 2436 Stansted to Krakow
- FR 7117 Stansted to Strasbourg
- FR 281 Stansted to Toulouse
- FR 5994 Stansted to Madrid
- FR 5993 Madrid to Stansted
- FR 33 Oslo Torp to Stansted