Britain's Civil Aviation Authority has told low-priced carrier Ryanair that it would have to reimburse approximately 700,000 ticket holders from the company's own pockets by 1700 BST on Friday or risk "enforcement action".
The authority has also instructed the airline to publicly state whether it will reimburse expenses for affected passengers.
"There is still no information here about how expenses will be treated where passengers are re-routed to and/or from other airports or where they otherwise incur additional out-of-pocket expenses as a result of the cancellations", the CAA said.More news: Moto X Play Android 7.1.1 update reportedly released in Brazil
"There are clear laws in place, which are meant to assist passengers in the event of a cancellation, helping minimize both the frustration and inconvenience caused by circumstances completely out of their control", CAA Chief Executive Andrew Hainess said on Thursday.
The regulator had threatened "enforcement action" toward Ryanair, primarily concerning the airline's failure to properly inform customers of their right to be rerouted with another airline in the event of a cancellation.
The airline has said the latest schedule changes - which affected popular routes including Stansted to Edinburgh, Gatwick to Belfast, and Glasgow to Las Palmas - will mean there is no new need for flight cancellations.More news: Will resign if asked to go on leave: BHU VC Tripathi
Passengers who have already accepted a refund must be offered a reimbursement of the difference with any higher fare.
All the new emails must be sent out by 5pm on October 4, after first being checked and approved by the CAA.More news: In Super Mario World, Yoshi was getting punched in the head
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