Ryanair has made a U-turn and agreed to move passengers whose flights it has cancelled onto other airlines if necessary.

The Civil Aviation Authority (CAA) has stepped in after Ryanair chief executive Michael O'Leary said the budget carrier would not pay for passengers to take flights on other airlines, stating that that was not required under EU flight delay law. This comes amid widespread disruption caused by Ryanair cancelling thousands of flights.

The CAA wrote directly to O'Leary on Monday and says it has confirmation Ryanair will use other airlines to help those affected, if required. The regulator has promised to "closely monitor" the situation, and will consider enforcement action if the airline does not comply.

Ryanair says a total of 2,100 flights have been cancelled, affecting 315,000 customers, which it blames on pilots being owed leave. It published a full list of cancelled flights on its website on Monday.

See our Ryanair cancels 100s of flights news story for a full rundown of your rights and what you can try if you're affected.

CAA 'concerned it is Ryanair's intention to fail to meet its obligations'

In his letter to Ryanair, Richard Moriarty, the director of consumers and markets at the CAA, said: "Whilst I appreciate that most Ryanair passengers will likely be re-routed on the same day on another Ryanair flight, many will not.

"This latter group of passengers may be better served by flying with a different airline (or indeed to or from a different nearby airport).

"The CAA's view is that Regulation EC261 requires Ryanair to offer passengers on cancelled flights alternative travel options, including flying with a different airline. Your statement yesterday runs contrary to our view and contradicts the assurances that we were given on this point by your legal team on a call at 10am yesterday morning.

"I am therefore concerned that it is Ryanair's express intention to fail to meet its obligations under Regulation EC261 and that this will give rise to significant consumer harm."

The CAA says Ryanair has since confirmed it will re-route passengers using other airlines.

A Ryanair spokesperson said: "We have re-accommodated over 175,000 customers on other Ryanair flights – over 55% of affected customers – and more than 63,000 flight refunds have been processed (over 20% of affected customers).

"Ryanair expects to have processed over 300,000 alternative routings or refunds for customers (over 95% of affected customers) by the end of this week, within six days of customers being notified of these flight cancellations."

When pushed on whether Ryanair has agreed to use other airlines, he added: "Given the fact we have re-accommodated 75% of our affected customers and given the size of our network, it's unlikely we will need to, but will do so if necessary."

What should I do if I need an alternative flight?

If your flight's cancelled, you can choose between a refund or an alternative flight.

If you opt for the alternative flight, but find it's not suitable, contact Ryanair and ask for a better option – using an alternative airline if necessary.

EU flight delay law says passengers whose flights have been cancelled must be offered "re-routing, under comparable transport conditions, to their final destination at the earliest opportunity".

It does not define exactly what "earliest opportunity" means, but if you're inconvenienced and feel it is reasonable to ask for a more suitable alternative flight on a different airline, you should do so.

For details on how to claim compensation, see Ryanair cancels 100s of flights – your rights.

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