Britain's Civil Aviation Authority (CAA) has launched enforcement action against Ryanair over its latest round of flight cancellations.
The carrier is extending its flight cancellation plan until mid-March next year.
The cancelled flights between November and March could affect up to 400,000 passengers.
No flights to and from the Republic of Ireland are affected, but a London to Belfast route is facing disruption.
The CAA claims the carrier has persistently misled passengers "with inaccurate information" about their rights.
Ryanair CEO Michael O'Leary
The aviation body says it wrote to Ryanair after its last tranche of cancellations seeking assurances around how and when they would reroute passengers onto alternative flights.
It told the airline to make a "corrective public statement, to ensure customers were not misled and had accurate comprehensive information relating to their rights and entitlements."
But the CAA says in announcing thousands more cancellations today, "the airline has again failed to provide customers with the necessary and accurate information relating to their passenger rights".
"We have now told Ryanair that we are expediting enforcement action against them", it adds.
In response, Ryanair says: "We are in correspondence with the CAA and have addressed their concerns".
