Ryanair has been threatened with legal action for “persistently misleading” passengers about their rights following thousands of flight cancellations.
The Civil Aviation Authority said it had launched “enforcement action” against Ryanair, the first step towards court action.
Ryanair was wrong to claim it did not have to re-route passengers on rival airlines, it said.
The action comes after Ryanair cancelled a further 18,000 flights.
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.
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 aviation authority said it had already written to Ryanair asking it to make a public statement to ensure customers were not misled after the first wave of flight cancellations.
But it said the airline had so far not complied with its request.
“There are clear laws in place, which are intended to assist passengers in the event of a cancellation, helping minimise both the frustration and inconvenience caused by circumstances completely out of their control.”
“We have made this crystal clear to Ryanair,” said CAA chief executive Andrew Haines.
Ryanair boss Michael O’Leary said it was “in correspondence with the CAA and have requested an early meeting to address their concerns.”
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.
Analysis: By Richard Westcott, 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.