(Reuters) - Ryanair RYA.I, Europe's busiest airline, was warned to deliver "action, not words" by British aviation authorities on Thursday after it failed to inform 400,000 customers of their full rights regarding canceled flights through Christmas.

Ryanair has more than doubled its unprecedented cancellations since last week, with cuts now affecting more than 700,000 customers over the next five months.

Ryanair has denied it has a pilots shortage and has blamed the wave of cancellations on pilot rostering problems.

The UK Civil Aviation Authority rebuked the Irish budget airline for providing “misleading information” after it offered affected customers refunds or alternative Ryanair flights.

The rules oblige Ryanair to also offer customers flights on other airlines if there is no suitable Ryanair service available.

The CAA also accused Ryanair of failing to inform passengers about its obligation to cover additional expenses incurred because of a canceled flight, such as hotels and meals.

“We will be meeting with the CAA and will comply fully with whatever requirements they ask us to,” Ryanair said in a statement on Thursday.

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CAA CEO Andrew Haines told Sky News on Thursday that the CAA was “furious” with Ryanair and that the regulator wanted to see “action, not words”.

LAST WEEK

Ryanair boss Michael O’Leary last week sought to address the airline’s problems with a first wave of 315,000 cancellations.

He told investors then that the impact would be less than 25 million euros and that Ryanair was finalising a bid for struggling Italian carrier Alitalia.

On Wednesday, that cost figure doubled and Ryanair abruptly dropped its planned Alitalia bid.

“We’ve made it clear their behavior is unacceptable... It’s only when we get to steps of court action very often that they are prepared to comply with the law,” the CAA’s Haines said.

The CAA has the power to seek legal undertakings from operators to make sure they comply with consumer rights law and to take court action if they fail to do so.

Ryanair shares were down 2 percent on Thursday. The stock gained more than 3 percent on Wednesday as analysts welcomed the company’s plan to restrict growth this winter and its decision to drop its planned bid for Alitalia.