Ryanair has been ordered by the UK's aviation regulator to sort out compensation for hundreds of thousands of travellers hit by mass flight cancellations by 5pm today.
The Civil Aviation Authority (CAA) 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 a letter from the CAA.
In addition, the Dublin-based carrier 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.
"They are not making it clear to people their entitlement", Mr Haines told BBC Radio 4's Today programme.
"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."
The latest round of cancellations includes several popular routes used by UK travellers, such as Gatwick to Belfast, Stansted to Edinburgh and Glasgow, 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.
Ryanair says the cancellations were brought about because of an error with pilot holiday rosters.
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.
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."