Britain’s competition watchdog said Thursday that it had opened an investigation into Irish carrier Ryanair over fees that parents must pay to sit with their children.

The no-frills airline requires at least one parent to sit with children aged two to 11 during a flight, but that means they must pay for what it calls a “mandatory family seat”.

The Competition and Markets Authority (CMA) is investigating whether the fee, typically around £8 ($11) per flight, is “unfair” under consumer law.

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For other passengers, reserving a seat is optional.

The CMA said it would determine whether Ryanair’s seat reservation fees mean parents are being charged to meet “child safety and disability-related obligations as set out under aviation rules”.

“The CMA understands that Ryanair is the only major airline flying out of the UK to impose this charge,” the agency added.

It also noted that Ryanair does not apply the fee on flights to and from Italy following action by the Italian civil aviation authority.

In response, Ryanair said its family seating policy “fully complies with all relevant laws and regulations”.

“Adults travelling with children pay one reserved seat fee, but can select reserved seats beside them for up to 4 children on the same booking free of charge,” the company said in a statement.

“Ryanair looks forward to disproving these false CMA claims during this bogus investigation,” it added.

The CMA said it expected to provide an update on the investigation within six months.

Infringing on consumer protection laws can lead to fines of up to 10 percent of a company’s global revenue.