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 on Friday.
The Civil Aviation Authority is launching enforcement action against Ryanair for "persistently misleading" passengers regarding their rights to compensation.
The regulator has asked for a meeting with the budget airline and warned it would take legal action "if necessary".
Ryanair has pledged to "comply fully" with the body.
It comes as the firm extended its list of flight cancellations into 2018.
Some 400,000 further customers will be affected when Ryanair axes 18,000 flights until mid March - on top of the 50 flights a day already scrapped for September, October and November.
From April next year, the number of routes will increase, but 10 fewer aircraft will be in operation than was planned.
The CAA has now sent a letter to the low-cost carrier explaining its intentions.
Ryanair was accused of falsely claiming it did not have to re-route passenger on other airlines, particularly when there are no other services available.
The CAA said 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.
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.
– ANDREW HAINES, CAA CHIEF EXECUTIVE
The flights have been axed after rostering errors meant the airline had allowed too many pilots to go on leave at the same time, meaning the company now has a shortage.
Taking more flights out of service means that the Irish carrier will be able to "roster all of the extra pilot leave necessary" for the rest of 2017.
Since the initial announcement was made, 2,000 flights have already been grounded, costing the airline almost £22 million.
Free flight vouchers for affected passengers are thought to cost the airline an extra £22 million.
Which routes will be axed from November 17?
- Edinburgh – Szczecin
- Glasgow – Las Palmas
- London (LGW) – Belfast
- London (STN) – Edinburgh
- London (STN) – Glasgow
- Newcastle – Faro
- Newcastle – Gdansk
- Hamburg – Edinburgh
- Bucharest – Palermo
- Chania – Athens
- Chania – Pafos
- Chania – Thessaloniki
- Cologne – Berlin (SXF)
- Gdansk – Warsaw
- Hamburg – Katowice
- Hamburg – Oslo (TRF)
- Hamburg – Thessaloniki
- Hamburg – Venice (TSF)
- Sofia – Castellon
- Sofia – Memmingen
- Sofia – Pisa
- Sofia – Stockholm (NYO)
- Sofia – Venice (TSF)
- Thessaloniki – Bratislava
- Thessaloniki – Paris BVA
- Thessaloniki – Warsaw (WMI)
- Trapani – Baden Baden
- Trapani – Frankfurt (HHN)
- Trapani – Genoa
- Trapani – Krakow
- Trapani – Parma
- Trapani – Rome FIU
- Trapani – Trieste
- Wroclaw – Warsaw
What should affected passengers do?
Affected passengers will have received an email.
They will be offered an alternative flight or a full refund.
Despite the high number of cancellations, only 1% of passengers have been affected.
Customers whose September and October flights have been cancelled will have received a £35 one-way or £80 return Ryanair travel voucher.
Under EU law, passengers given less than 14 days notice of a flight cancellation are entitled to claim compensation worth up to 250 euros (£221) depending on the timing of alternative flights and if the issue was not beyond the responsibility of the airline, such as extreme weather.
Speaking last week, Michael O'Leary, chief executive of the budget airline said: "If they're [customers] not satisfied with the alternative flights offered they can have a full refund and they will all be entitled to their EU261 compensation entitlements.
"We will not be trying to claim exceptional circumstances.
"This is our mess-up. When we make a mess in Ryanair we come out with our hands up.
"We try to explain why we've made the mess and we will pay compensation to those passengers who are entitled to compensation, which will be those flights that are cancelled over the next two weeks."
What has Ryanair said?
In a statement, Mr O'Leary "sincerely apologised to those customers who have been affected by last week's flight cancellations, or these sensible schedule changes announced today.
"From today, there will be no more rostering related flight cancellations this winter or in summer 2018.
"Slower growth this winter will create lots of spare aircraft and crews which will allow us to manage the exceptional volumes of annual leave we committed to delivering in the nine months to December 2017.
"We will start a new 12 month leave period on the 1st of Jan 2018 in full compliance with EU regulations and the IAA's requirements."
The firm also said that it has scrapped plans to bid for bankrupt Italian airline Alitalia in order to "focus on repairing this rostering problem this winter".
"Ryanair will eliminate all management distractions starting with its interest in Alitalia," the group said.
What is next for Ryanair?
The firm 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".
But the airline will also face the Civil Aviation Authority's enforcement action for misleading passengers on compensation, meaning the budget airline could be facing a very expensive bill.
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