Britain’s Civil Aviation Authority (CAA) is launching "enforcement action" against Ryanair for failing to give customers accurate information about their rights following a wave of flight cancellations.
The regulator has asked for a meeting with the airline as part of a consultation that will last at least seven days and could take legal action for breaching consumer protection laws "if necessary".
The CAA sent a letter to the low-cost carrier explaining its decision.
It said the company falsely claimed it did not have to re-route passengers on other airlines, particularly when there are no other services available.
The regulator added that Ryanair also stopped short of providing details on its obligations to refund additional expenses incurred by passengers as a result of cancellations and re-routing.
Those expenses include meals, hotels and transfer costs, the CAA explained.
It failed to correct that information through a public statement despite CAA requests to do so earlier this month, having already seen about 2,000 flights grounded after the company miscalculated pilot leave.
The company has since cancelled an extra 18,000 flights for the winter season in a move that will hit 400,000 customers.
The regulator said it was concerned Ryanair was breaching consumer protection laws by withholding that information.
CAA chief executive Andrew Haines said: "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, who are well aware of their legal obligations, which includes how and when they should reroute passengers, along with the level of information it provides its passengers.
"The information Ryanair published today again fails to makes this clear.
"In expediting our enforcement action, we are seeking to ensure that Ryanair's customers will receive the correct and necessary information, to make an informed choice about an alternative flight."
In a statement this evening, Ryanair said it is "in correspondence with the CAA" and has "addressed their concerns".
Ryanair said this afternoon it was suspending 34 routes for the winter season, spanning from November to March 2018.
It adds to mounting anger against Ryanair, which has come under heavy fire after recently shelving up to 50 flights every day for six weeks.
Ryanair Chief Executive Michael O'Leary has blamed the move on mismanagement of pilots' annual leave, leading to the over-allocation of blocks of holidays.
Ryanair said the latest step will "eliminate all risk of further flight cancellations" and remove the risk of similar problems recurring next year.
Mr O'Leary said in a statement: "We sincerely apologise to those customers who have been affected by last week's flight cancellations or these sensible schedule changes announced today."
The firm also plans to roll out a series of low-fare seat sales for winter 2017 as it is "confident that there will be no further roster-related cancellations".
It argued less than 1% of the 50 million customers Ryanair will carry this winter are impacted and all affected passengers have received an email alerting them and offering alternative flights or full refunds.
They have also received a €40 travel voucher.
The flight cancellations have so far cost the airline around €25m.
Alex Neill, managing director of Which? home and product services, said: "This situation is a complete and utter shambles that now extends to up to nearly three-quarters of a million people.
"The news means that for some of its passengers, Ryanair has effectively cancelled Christmas.
"On top of leaving the travel plans of many in tatters, offering a €40 voucher will be cold comfort for passengers that can't book the flight they need."