The pair, who has been romantically involved for six years, had been rumoured to be headed to the Alps.
The two, instead, landed at the Prince Charles's Aberdeenshire hideaway at Birkhall on New Year's Day where they would be with him and Camilla for six days.
"This is the first time Wills has taken Kate away with his father. It has certainly caused some excitement," the Sun quoted a source as saying.
The lovers had previously been speculated to have relationship trouble following William's decision to prioritise his career in the Royal Air Force, which had led them to put their alleged wedding plans on hold for eight years.
The 26-year-old, who is to spend 18 months training to be a pilot from January, followed by another six-year commitment in the RAF, is said to have decided not to marry before completing his training to become a Search and Rescue Officer. (ANI)