Passengers may be paying up to £390m a year in extra airline charges to sit together as a family or a group, the UK's aviation watchdog has warned.
The Civil Aviation Authority (CAA) said it would launch a review to see if the levies were applied in a "transparent way" after customers complained that the fees varied from carrier to carrier.
Just over half the passengers who travelled in groups had to pay extra to sit together, said the CAA after interviewing 4,000 air travellers in a survey that concluded last month.
The watchdog said there was confusion about various airlines' group flying policies.
Overall, 18 per cent of passengers were separated from their travelling companions if they did not pay extra to be seated together.
On Ryanair, this figure rose to 35 per cent, the highest among all the airlines surveyed. Emirates was second highest with 22 per cent and Virgin Atlantic was third with 18 per cent.
Last summer Ryanair came under fire over the frequency with which its passenger groups who did not pay extra were split up - even when flights were relatively empty.
This prompted passengers to complain that the levies were part of the hidden charges some airlines pass on at the last minute before a booking is completed.
Ryanair denied that it had a policy of deliberately separating fliers who did not pay the extra charge.
Passengers are typically charged between £5 and £30 per seat extra by carriers. The CAA estimated that that brought in between £160m and £390m.
CAA chief executive Andrew Haines said: "Airline seating practices are clearly causing some confusion for consumers. Airlines are within their rights to charge for allocated seats but if they do so it must be done in a fair, transparent way.
"Our research shows that some consumers are paying to sit together when in fact they might not need to."
He added: "We will be looking into how airlines decide where to seat passengers that have booked as part of a group and whether any airlines are proactively splitting up groups of passengers when they could be [sitting] together. We will not hesitate to take any necessary enforcement action should it be required at the end of the review."
The watchdog will complete its review in March and announce its findings during the summer.