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HAVE you ever booked a flight with friends or family only to realise you've been split up all over the plane? Better pay extra to ensure you're all seated together, right?

This frustrating scenario is becoming lucrative for airlines around the world, who are cashing in on passengers' desire to sit with their loved ones.

Now, the Civil Aviation Authority (CAA) in the UK is investigating this practice to find out whether some airlines are deliberately giving families separate seats on flights so they end up paying more to sit together.

The watchdog found more than a third of UK families were being seated separately, costing passengers a whopping extra $691 million a year to ensure they end up together.

It comes after furious passengers complained about the practice, posting images on social media showing them being split up despite there being rows of empty seats available.

What's more, some airlines are profiting from confusion around the seating situation.

The CAA said it wanted to make sure seat allocation practices were "fair and transparent" and has written to 20 airlines to demand an explanation about how their computer systems allocate seats.

"Airline seating practices are clearly causing some confusion for consumers," CAA chief executive Andrew Hinton said.

"Our research shows that some consumers are paying to sit together when, in fact, they might not need to. We will not hesitate to take any necessary enforcement action should it be required at the end of the review."

In Australia, there are no guarantees when it comes to seating. According to consumer watchdog Choice: "Most airlines say they can't guarantee any seat and reserve the right to change seating, even after you've boarded the plane."

The fees can definitely add up. For example, for those flying on Qantas it can cost up to $35 per person to pre-book a seat, while it can cost up to $25 per person at TigerAir.