Ryanair has been accused by the aviation regulator of failing to respond to its request to discuss how it is treating passengers whose flights have been cancelled.
The Civil Aviation Authority (CAA) has told the Dublin-based carrier it has until 5pm on Friday to sort out compensation arrangements for hundreds of thousands of affected travellers or face possible action.
On Wednesday, the CAA published a letter inviting Ryanair to attend a meeting or take part in a conference call to explain how it will stop breaching consumer law by not fully informing passengers of their rights.
The regulator issued a statement on Friday which said: "As part of our enforcement action we have requested a meeting with Ryanair to discuss our ongoing concerns around passenger rights.
"As yet we have received no response to this request, nor has Ryanair proposed any other meetings, whether in person or via telephone."
Speaking before the CAA published its latest statement, a Ryanair spokesman said: "We will be meeting with the CAA and will comply fully with whatever requirements they ask us to."
The CAA has instructed the budget airline to tell passengers they are entitled to be re-routed by another carrier, and explain how that will work.
Ryanair must also publicly state it will reimburse expenses for affected customers, according to the CAA.
In addition, Ryanair must commit to helping passengers who chose an unsuitable option as a result of being misled.
It comes after the regulator accused the airline of "not complying with the law" over its handling of the fiasco.
CAA chief executive Andrew Haines said he was "furious" after Ryanair cancelled an extra 18,000 flights for the winter season on Wednesday - a move that will hit 400,000 customers.
A Ryanair internal memo allegedly instructed call centre staff to offer flights with other carriers - provided the price "does not exceed three times the value of the original Ryanair fare".
The document, unearthed by ITV News, was criticised by consumer group Which?
Managing director Alex Neill said: "Ryanair appears to be plucking figures out of thin air as there is no legal basis for the arbitrary figure they've set.
"The law says passengers must be rerouted and there's no specified limit on cost. This yet again highlights the importance of the action which the Civil Aviation Authority has started.
"It must force Ryanair to immediately change its behaviour and comply with the law."
The latest round of cancellations includes several popular routes used by British travellers, such as Stansted to Edinburgh and Glasgow, Gatwick to Belfast, Newcastle to Faro, and Glasgow to Las Palmas.
It adds to mounting anger against Ryanair, which was already coming under heavy fire after cancelling up to 50 flights a day earlier this month.
Passengers have expressed their frustration with the airline, with many left out of pocket due to a lack of alternative flights and accommodation bookings they can no longer use.