The Competition and Markets Authority opened an investigation into Ryanair over charges that typically cost £8 each way for a parent to sit next to a child.
The watchdog said Ryanair's terms require a parent to sit with children aged 2 to 11, raising concerns the airline may be charging families to meet child-safety and disability-related obligations under aviation rules.
Ryanair rejected the case as "bogus" and said its policy complies with the law because adults pay for one reserved seat while up to four children's adjacent seats on the same booking are assigned free.
The CMA will now assess whether that seating approach breaches consumer law, testing how airlines can price seat reservations when family seating is effectively mandatory.