Britain's Civil Aviation Authority says it is beginning a review into airlines' seating policies, including whether some operators are deliberating splitting up groups of passengers travelling together.
Some customers have complained that they have been seated away from their friends or family in order to force them to pay extra for allocated seating.
"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," said Civil Aviation Authority (CAA) chief executive Andrew Haines.
"We will not hesitate to take any necessary enforcement action should it be required at the end of the review."
In a survey of passengers from 10 airlines conducted by pollster YouGov released on Saturday, those who flew with Irish low-cost airline Ryanair were most likely to report being separated from their group when not paying more to sit together.
Ryanair said it was happy to participate.
"Our policy is very clear for our customers and seats can be purchased from just 2 euros and kids travelling in families get free seats," said a spokeswoman.
Rival easyJet said it would co-operate with the CAA review and that it tried to keep groups together.
"Unlike some airlines, if passengers choose not to pay to select their seats, easyJet's seating system is programmed to try and seat families together when they check-in online by using an algorithm," the firm said in a statement.