The seats are popular with travellers as they provide additional leg room, which in turn has encouraged Ryanair to charge passengers for the privilege of sitting there. Fliers occupying those seats are also expected to follow directions and -- if necessary -- open the doors in the event of an emergency.

However, due to some passengers refusing to pay the extra charge, many Ryanair flights have taken off with those seats vacant. In those cases, passengers in the surrounding rows, further from the exit, are asked to familiarise themselves with the evacuation procedure.

Some of those passengers have reportedly expressed doubts about being able to understand the instructions and open the emergency doors if they are not sitting next to the exit.

The Irish Aviation Authority (IAA) has launched an investigation into the issue, while the Civil Aviation Authority and the British Airline Pilots Association (BALPA) have also expressed concerns.

"Our guidance to UK-registered airlines is that whoever is sitting next to the emergency exit must be briefed about what to do," said a spokesman for the CAA. "If that person says they are not willing to do it, then someone else must be found who is happy with that role."

He added: "It's an important task. It's not easy to open the doors and they must be physically strong enough to throw them from the plane."

A spokesman for Ryanair said it does not believe the issue will be a problem, but said the airline would continue to cooperate with the IAA.

"Ryanair complies with all mandatory safety directives," he added. "All passengers are provided with the same safety and evacuation information."

A number of other airlines registered in Britain also charge passengers extra to sit next to the emergency exit.