Ryanair is facing enforcement action by the airline regulator, after it refused to compensate passengers for disruption caused by staff taking strike action.
The Civil Aviation Authority said the low-cost airline has rejected compensation claims.
The CAA said it does not believe the strikes are "extraordinary circumstances", as Ryanair has claimed, and are therefore not exempt, meaning its customers should be compensated.
In October the commercial court in Barcelona ruled that no compensation should be paid by Ryanair in the wake of strikes.
A Ryanair spokesman said: "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."
'Shirk its responsibilities'
Pilots of Ryanair during a 24-hour strike by the pilots on August 10, 2018 in Frankfurt, Germany.
Ryanair's flights were hit by widespread strikes over the summer by the carrier's pilot and cabin crews, but it also suffered amid the industry-wide air traffic control industrial action that saw thousands of flights cancelled across Europe.
Rory Boland, editor of magazine Which? Travel, said: "Customers would have been outraged that Ryanair attempted to shirk its responsibilities by refusing to pay out compensation for cancelling services during the summer - which left hard-working families stranded with holiday plans stalled.
"It is right that the CAA is now taking legal action against Ryanair on the basis that such strikes were not extraordinary circumstances and should not be exempt, to ensure that the airline must finally do the right thing by its customers and pay the compensation owed."
The CAA said passengers with an existing claim will now have to wait until the outcome of its enforcement action against the airline.
The watchdog has made repeated calls for Ryanair to compensate passengers affected by staff strikes.
Airlines can refuse to pay out for "extraordinary circumstances", which normally involves bad weather or air traffic controller strikes.