Ryanair has been accused by the UK’s aviation regulator of “not complying with the law” over its handling of flight cancellations.
Andrew Haines, chief executive of the Civil Aviation Authority, said he was “furious” with the Dublin-based carrier.
The CAA is threatening to take Ryanair to court because it is not telling passengers they are entitled to be rerouted by other airlines.
It is believed to be the first time the regulator has issued such a warning to a major airline over the issue in recent years.
Mr Haines said: “They are not making it clear to people their entitlement. If they follow through on what they are saying, then they would be breaking the law.”
A Ryanair spokesman said: “We will be meeting with the CAA and will comply fully with whatever requirements they ask us to.”
On Wednesday, the airline cancelled an extra 18,000 flights for the winter season in a move that will hit 400,000 customers, including several popular routes used by British travellers, such as Stansted to Edinburgh and Glasgow.
The move adds to mounting anger against Ryanair, which was already coming under heavy fire after cancelling up to 50 flights a day earlier this month.
Ryanair says the cancellations were brought about because of an error with pilot holiday rosters.
Passengers have expressed frustration with the airline, with many left out of pocket due to a lack of alternative flights and wasted bookings for unused accommodation.
Mr Haines said airline passengers are “well-protected by the law”.
He went on: “They are entitled to compensation and if there is a cancellation, they are entitled to be rerouted by other airlines.
“The chief executive of Ryanair (Michael O’Leary) has gone on record and said he is not going to do that. He then issued a clarification.
“But yesterday, when they announced 18,000 further cancellations, they failed to follow through on that.
“We are furious they are not complying with the law and they are not giving customers what they are entitled to.”
The CAA has asked for a meeting with the airline as part of a consultation that will last at least seven days.
It could also take legal action against Ryanir over possible breaches of consumer protection laws.
It says Ryanair has falsely claimed it did not have to reroute passengers on other airlines, particularly when there are no other services available.