Ryanair PLC (LON:RYA) is facing legal action over its failure to compensate customers for disruptions caused by strikes during the summer.
The Civil Aviation Authority will bring the case and says people are entitled to compensation under EU law.
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Ryanair says the delays were the result of extraordinary circumstances, so it does not have to pay.
A statement from the CAA said: "The UK Civil Aviation Authority has today started enforcement action against Ryanair, following the airline's decision that financial compensation is not payable under European Commission Regulation 261/2004 for flight disruption resulting from industrial action by the airline's staff this summer.
"Ryanair passengers have made claims for compensation directly to the airline, but these have been rejected.”
Ryanair has also withdrawn from AviationADR, the arbitration service set up to handle customer complaints said the CAA.
The airline said: “Courts in Germany, Spain and Italy have already ruled that strikes are an ‘exceptional circumstance’ and EU261 compensation does not apply.
“We expect the UK CAA and courts will follow this precedent.”
Shares in the airline were little changed at €11.44.