Originally the 27-year-old was meant to join Kate and her family for two weeks in the Caribbean over the summer but his commitments to the RAF have meant he will instead be transferred to Wales for a month-long course with search and rescue teams.
With its white sand beaches, coral reefs and palm and coconut trees, Mustique is a popular destination with the rich and famous – it was a favourite of Princess Margaret and has also attracted the likes of Mick Jagger, Kate Moss and David Bowie.
The royal couple, who met at St Andrews University in 2001, have visited together on several previous occasions.
And while he is missing out on the chance to soak up the sun - Mustique enjoys balmy temperatures all year around – William is believed to be looking forward to temporarily moving from his current base at RAF Shawbury in Shropshire, where he is currently training to be a search and rescue pilot, to Anglesey's RAF Valley next month.
"It's an important part of Prince William's training and he is looking forward to it immensely," a friend confirmed.