The Civil Aviation Authority has launched a review of airline allocated seating policies after consumers said the current policy is confusing.
Part of the investigation will focus on whether airlines are pro-actively splitting up groups in order to get more money.
The CAA said some consumers are paying to sit together when they may not need to do so and UK passengers are collectively paying up to £390 million a year on allocated seating.
Just over half of consumers said their airline informed them before they booked their flight that they would need to pay to ensure their group could sit together
Ten percent of respondents said that they had been informed after they booked and a further 10% said that they were never made aware by their airline that they may need to pay more to guarantee sitting together
The research, among over 4,000 consumers who have flown as part of a group (of two or more people) in the past year, also found that, although most were aware that they might not be able to sit together if they booked as a group, almost half believed their airline would automatically allocate seats together.
Around half of all passengers who sat together did not have to pay an additional charge to do so. However 7% said that they had to change seats either at check-in or on-board to avoid being sat apart.
Different airlines behave differently, as consumers flying with some airlines were more likely to report being separated from their group than others.
On average, 18% who didn’t pay more to sit together were separated from their group, yet the figure rose to 35% of Ryanair passengers, 22% of Emirates passengers and 18% of Virgin Atlantic passengers.
Of the other major airlines, 16% of Jet2 passengers who didn’t pay more were separated; 15% of passengers with BA, easyJet and Thomas Cook reported the same, as did 12% of Flybe and TUI Airways passengers.
Almost half of respondents (46%) felt negatively towards the airline when they realised they would have to pay more to guarantee sitting together.
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.
"It also suggests that consumers have a better chance of being sat together for free with some airlines than with others. The research shows that it is the uncertainty around whether their group will be split up by the airline that is driving consumers to pay for an allocated seat.
"Findings from our research show that UK consumers collectively may be paying between £160-£390m per year for allocated seating.
"Of those paying, two-thirds spent between £5 and £30 per seat and a further 8% paid £30 or more. Our work will consider whether or not these charges are fair and transparent.
"As part of the review, we will be asking airlines to provide information on their policies and practices.
"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 pro-actively splitting up groups of passengers when, in fact, they could be sat together.
"We will not hesitate to take any necessary enforcement action should it be required at the end of the review."
