Irish investigators are examining whether budget carrier Ryanair's scheme to charge passengers to reserve emergency exit seats contravenes the country's safety regulations.
Under the scheme Ryanair allows passengers to reserve the more spacious seats - including those in the two overwing exit rows - for a €10 ($13) fee.
The Irish Aviation Authority wants to ascertain whether this scheme results in exit rows being left vacant, and a source familiar with the situation confirms that the authority is looking into the matter after it was "brought to its attention".
In a policy document on self-help emergency exits, dated 31 January, the authority states that seats immediately adjacent to such exits should be occupied during taxi, take-off and landing.
"This will ensure a speedy and efficient response should a situation arise which warrants an emergency evacuation," it says.
Ryanair operates a fleet of Boeing 737-800s which are configured with 189 seats and have four emergency exits located in seat rows 16 an 17.
The airline has not indicated whether, in the event of exit-row seats not being reserved, it reassigns passengers to occupy the vacant places.
"Ryanair complies with all mandatory safety directives," the carrier states.
Several airlines have introduced similar schemes to charge passengers for the additional space offered by exit-row seats.