The UK's aviation authority said it would take action to force Ryanair to pay compensation to customers affected by strikes held by its staff this summer.
The Civil Aviation Authority (CAA) said the strikes were not exempt from EU rules on compensation and it had started enforcement action against the airline.
Ryanair cabin crew and pilots have staged a number of strikes this year, forcing the airline to cancel hundreds of flights, after it recognised unions for the first time in 2017.
Passengers claiming compensation will not have to await the outcome of the enforcement action, the CAA said.
Enforcement action is when the CAA seeks legal undertakings from operators to ensure they change their policies and comply with the law.
"Courts in Germany, Spain and Italy have already ruled that strikes are an 'extraordinary circumstance' and EU261 compensation does not apply. We expect the UK CAA and courts will follow this precedent," a spokesman for Ryanair said.
Analysts at Goodbody also noted that rival airlines Lufthansa and IAG's Aer Lingus had not been obliged to pay out EU 261 compensation due to strikes their staff had recently held.
Passengers booked on flights which are delayed or cancelled may be entitled to compensation. Here are some of the key questions around the claims process:
What rules apply?
EU law protects passengers whose flights are delayed or cancelled under Regulation 261.
This applies to flights either departing from an EU airport or those that are both arriving at an EU airport and operated by an EU airline, such as Ryanair.
Why is the cause of disruption important?
Compensation is payable when an airline is deemed to be at fault, such as the crew being late, cancellation due to under-booking or an aircraft component failure.
If a carrier can prove disruption was caused by "extraordinary circumstances" - typically severe weather, air traffic control strikes or a security risk - then it is not liable.
How large are payouts for delays?
Compensation ranges from €300 for short-haul flights, such as Edinburgh to Dublin, which are delayed by more than three hours, to €600 for long-haul trips, such as London to New York, which are delayed by more than four hours.
What about cancellations?
Compensation may be payable if a flight is cancelled within two weeks of departure.
Payouts vary between €125 euros and €600, depending on how much notice is given, the distance travelled and the timing of the alternative flight taken.
Is compensation automatic?
No, disrupted passengers must write a letter of complaint to the airline. They should keep as much evidence as they can, such as boarding cards and receipts to claim expenses.
Additional reporting PA