Ryanair has been threatened with legal action for "persistently misleading" passengers about their rights following thousands of flight cancellations.
The Civil Aviation Authority (CAA)'s chief executive Andrew Haines said he was "furious" that the airline was not complying with the law.
The authority has launched "enforcement action" against Ryanair for wrongly claiming it did not have to re-route passengers on rival airlines.
Ryanair now says it will co-operate.
In a brief statement, the airline said: "We already comply fully with all EU261 legislation, are meeting with the CAA and will comply fully with whatever requirements they ask us to."
The action comes after Ryanair cancelled a further 18,000 flights.
The CAA's Mr Haines told the BBC that he very much doubted the dispute would get as far as the courts, but added it was "unacceptable" that Ryanair was disregarding the law and customers' rights.
"These are simple things to fix and they're choosing not to fix them," he said. "People shouldn't have to choose between low fares and legal rights."
Mr Haines singled out Ryanair boss Michael O'Leary for particular criticism, telling Radio 5live: "Michael himself said he wasn't going to pay for passengers to fly on other airlines. That's against the law.
"My concern with Ryanair, and the reason we are speaking so openly, is they say one thing and yet they don't follow it through."
The fresh round of flight cancellations will be between November and March and affect the travel plans of a further 400,000 customers.
A total of 34 routes will be suspended this winter, including Stansted to Edinburgh and Glasgow, Gatwick to Belfast and Newcastle to Faro.
Earlier this month, the airline cancelled up to 50 flights a day through to the end of October, also affecting 400,000 passengers.
The regulator said that on both occasions Ryanair had failed to provide customers with "necessary and accurate" information about their rights.
Your rights explained
- Ryanair has about 24 hours after telling affected passengers of a cancellation to book them on an alternative Ryanair flight
- If the alternative Ryanair flight is not soon enough, or at a time that does not work for you, then you can tell Ryanair to book you on the most appropriate flight on an alternative airline at no extra cost to you
- Consumer group Which? says that it is best to make Ryanair do the legwork - rather than take a refund from the Ryanair flight, rebook yourself on another airline, and try to reclaim any extra cost from Ryanair if the new flight is more expensive. By doing so, you will probably only get a refund for the cancelled Ryanair flight, but not the difference for the more expensive alternative flight
- As more than two weeks' notice has been given for the new wave of flights cancellations, affected passengers will not be entitled to the extra compensation that is paid under EU rules for last-minute cancellations
The CAA said information provided on Ryanair's website failed to make it clear that the airline was obliged to refund all expenses incurred as a result of the flight cancellation.
Those expenses included meals, hotels, as well as transfer costs to re-route passengers on other airlines when there was no suitable alternative, the CAA said.
The airline has said that passengers affected by the move will be offered alternative flights or full refunds and had been emailed about advising them of flight changes occurring until the end of October.
They will also be offered vouchers of 40 euros (£35) one way, or 80 euros return, towards alternative flights on top of any refund.
Ryanair has blamed the series of flight cancellations on "messing up" pilot holiday rosters.
By Richard Westcott, BBC transport correspondent
This is the regulator firing a shot across the bows of Ryanair.
It doesn't feel that the airline is doing enough to tell people about their rights under EU law if their flight has been cancelled.
They could be entitled to money and a flight back with another airline if Ryanair can't help them get home in reasonable time.
The CAA will take views for the next seven days before deciding whether to press ahead with action.
It could end up in court, with Ryanair facing a fine, but what normally happens is the airline makes some changes and the CAA backs off.