Critics commented on the fact he'd attended just two royal engagements this year, despite only working 80 hours a month (an average of 20 hours a week) in his part-time role as an air ambulance pilot - with some going as far as to accuse him of being 'lazy'.
Sources at Kensington Palace initially defended the Duke, stating that the Civil Aviation Authority has rules about rest periods during work shifts, meaning that William must work a four days on, four days off shift pattern, and claimed that he was not allowed to perform other work on those rest days.
However, the Civil Aviation Authority later responded and said the palace was 'confused'.
Prince William started working as a helicopter pilot for the East Anglian Air Ambulance in July 2015
'It's true that you can only work a certain number of hours in a given period, but to suggest that pilots can't do anything else on some rest days is totally unenforceable,' a spokesperson for the CAA explained.
'When they are having rest days, their time is their own, and they can do what they want, including carrying out royal duties. We check pilots' shift patterns and the Duke is fully complying with CAA rules within the rota he is working, so his days off are his own.'
According to the CAA, the only thing pilots can't do on their days off is fly aircraft.
HELLO! magazine then reached out to the palace for further comment on the matter, and were told, 'Over the course of the year, the monthly average would be 80 hours on shift. His royal and charitable duties are on top of that.'
'There are engagements and tours and meeting and all sorts, and his team works with the Air Ambulance Service to fit it into his diary. This is no different to what he's been doing since he started flying. It's ultimately a very skilled and rewarding job - he's flying doctors around to help save people's lives.'
William juggles his work as an air ambulance pilot with his royal duties and life as a husband and father-of-two
In 2015, William made 87 public appearances in Britain and 35 overseas, during a busy year that also included the birth of his second child, Princess Charlotte.
During a recent meeting, a spokesperson for the East Anglian Air Ambulance charity said the Duke was 'very much part of the team' and was doing 'a very important job'.