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Long-running tensions between Europe’s biggest online travel companies and Ryanair are rising after the booking agents called on UK regulators to investigate the low-cost carrier’s data requirements for passengers who do not book directly through its website.

A group of companies including Booking.com, Expedia and Skyscanner wrote to the UK government and regulators on Wednesday to raise concerns over Ryanair’s “invasive, unnecessary and unfair” treatment of customers.

In the letter to government agencies including the Civil Aviation Authority and Information Commissioner’s Office, the companies said Ryanair forces many customers booking through third-party websites to hand over significant personal information in a complex process, including facial verification, in order to manage their booking or check-in online.

“Ryanair may be breaching UK data protection rules which require data collection to be necessary and kept to a minimum, which is clearly not the case with these additional checks,” the companies said in a letter, seen by the Financial Times.

The online travel companies said they “assumed” Ryanair’s “real motivation” for the checks, which are only applied to customers booking with travel agents, is to push passengers into booking directly with the airline the next time they travel.

Ryanair said its goal is “solely to be able to communicate with passengers directly . . . in order to comply with our legal obligations and to allow passengers to correct their contact details so that we can meet any customer needs that may arise”.

The airline said it does not have access to customers’ email addresses or payment details if they book through an online agent. This then forces the airline to verify their identities in compliance with regulatory protocols.

Ryanair added that its business model is “dependent” on direct online sales to customers, and that “many” online travel agents “mis-sell” its flights, including adding mark-ups such as inflated baggage or seat fees.

The claim and counter claim are the latest in a battle between Europe’s largest airline and some of the region’s leading online booking agents.

Ryanair has long criticised third-party booking companies for preventing it from dealing directly with customers and has called on all passengers to book through its own website.

Low-cost airlines prefer to deal with customers directly as they can sell them extra services — from car hire to seat bookings and luggage space — during the booking process.

These “ancillary” revenues are of critical important to Ryanair’s business model, which is based on offering passengers the cheapest possible fare and charging for extras. The airline made €1.18bn from ancillaries in its most recent quarter, a third of its overall revenue.

The online travel companies said no other airline requires customers to go through these checks and they had seen “significantly increased communication from customers affected by this issue after booking Ryanair flights with them”.

“Consumers are being caused undue stress and uncertainty over their travel plans during the peak summer period,” the companies said in the letter.

The ICO, the independent regulatory body for data protection, said it “recognised the concerns raised” over Ryanair’s collection of data, as well as “the stress and uncertainty experienced by customers worried about their travel plans.”

“We will consider the issues raised in the letter and will be making inquiries with Ryanair,” the agency said.

The CAA and Competition and Markets Authority declined to comment.