The airline never admitted to splitting up passengers deliberately and said that those who didn't pay for a chosen seat were "randomly" allocated one.
Research published by the British Civil Aviation Authority (CAA) in October found the likelihood of being split up varied between airlines – and found that it was most likely to happen on Ryanair, where 35 per cent of those surveyed were separated after opting not to pay for allocated seating.
The practice has also been reported by British Airways passengers.
In October, lawyer Jo Lightning tweeted: "BA charging us £39 to enable us to sit next to our 14-year-old despite a row of seats being available. Very poor."
The Daily Mail also reported that a couple on a transatlantic flight were automatically allocated seats on different rows and had to pay £100 to sit next to each other – even though the seat was unoccupied anyway.
BA's website states: "To get the best choice of seats and [to] make sure your family sits together, it's best to reserve your seats in advance, for which you might have to pay."
However, the company does state that children under 12 will not be seated without a parent.
According to a report by the Royal Aeronautical Society Flight Operations Group (FOG), splitting up families could also hinder emergency evacuations.
In the report, Emergency Evacuation of Commercial Passenger Aeroplanes, one of the 17 recommendations to improve evacuations was "Passenger seat allocation".
"This is especially important when adults and their children need to be seated near to each other if an emergency situation occurs, such an evacuation, decompression or air turbulence, when the assistance and supervision of an adult is likely to be of paramount importance," it reads.
"The UK Civil Aviation Authority decided that the seating of family groups should be such that family members are not seated remotely from each other, since group members who are separated might seek each other out in an emergency evacuation, which might have a serious impact on passenger flow to emergency exits."