Ryanair is under investigation by the UK’s competition watchdog over claims that parents may be unfairly charged to sit beside their children on flights.

The Competition and Markets Authority has opened an inquiry into the airline’s family seating policy, which it says can typically add around £8 each way to the cost of a booking. The watchdog will examine whether Ryanair’s approach is fair under consumer law and whether the total cost is made clear enough during the booking process.

According to the CMA, Ryanair’s terms state that an adult must sit with children aged between two and 11. The airline arranges this through what it calls a “mandatory family seat”, for which the accompanying adult is charged.

The regulator said its investigation will consider whether parents are effectively being made to pay for the airline to meet child safety and disability-related obligations set out under aviation rules. It stressed, however, that the inquiry is at an early stage and that no conclusion has been reached on whether the law has been broken.

Ryanair has strongly rejected the investigation, describing it as “bogus” and insisting that its family seating policy fully complies with all relevant legislation. The airline said adults travelling with children pay for one reserved seat, while up to four children on the same booking can be allocated reserved seats beside them free of charge.

The CMA said it understood Ryanair was the only major airline flying from the UK to impose this type of charge. Other carriers, it said, either seat children next to a parent or guardian without a fee, or allocate seats together automatically during booking.

The watchdog will also look at whether the charge is “dripped” into the booking process rather than being presented clearly at the start.

Hayley Fletcher, the CMA’s director of consumer protection, said extra charges could quickly increase the cost of a family holiday. She said businesses had been warned to show customers the full price upfront, adding that firms failing to do so faced possible enforcement action.

The UK Civil Aviation Authority says airlines must include all compulsory charges in advertised ticket prices, while optional extras should be clearly shown during booking. It also states that carriers should aim to seat children under 12 in the same row as an accompanying adult, or no more than one row or aisle away.

Consumer group Which? welcomed the investigation. Its travel editor, Rory Boland, said the organisation had repeatedly criticised Ryanair’s approach to family seating and urged the airline to stop charging parents before the investigation concludes.

The probe forms part of the CMA’s wider work to tackle cost-of-living pressures. Under new powers, companies found to have breached consumer law can face fines of up to 10% of global turnover.