Ryanair has cancelled thousands of tickets booked through price comparison websites.

Although the move has been described by travel industry body ABTA as "an appalling way to treat customers", the airline claimed that the tickets in question were sold "illegally".

A representative for Ryanair said it would continue to take action against sites that "have breached the terms and conditions".

The affected websites, such as Atrapalo and Edreams in Spain and Vtours in Germany, allow people to buy tickets without directly visiting the Ryanair website.

The attraction for travellers is that these sites tend to offer discounted fares. It is thought that bookings through such sites account for around 0.5 per cent of Ryanair's bookings.

Ryanair said it would compensate the websites, who in turn would be responsible for informing clients of the cancellations and refunding money.

Although the airline would not say which companies it will continue to target, it said the websites were aware of the issue.

It also seems some UK travellers are safe. Travelsupermarket and Skyscanner, two of the UK’s most well known price comparison sites, have both issued statements to reassure users that the tickets they have sold would be valid.

However, ABTA said the move by Ryanair would not be good for relations in the travel industry.

Sean Tipton, a representative for the organisation, said: "This has never happened before. Airlines and travel agents work closely together and have done for years."

But the Office of Fair Trading said the airline was within its rights as long as it had " put in place proper systems to inform passengers that their flights have been cancelled."

"Whether to accept bookings via intermediaries is a commercial decision for Ryanair," it said.

It advised customers who may face difficulties due to a cancelled ticket to contact Consumer Direct.