Speaking to ABC News in America about his nephew Prince George, James said: "I just want to be the best uncle possible, like any uncle wants to be."
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The 27-year-old continued: "The cool uncle, the fun uncle!". Asked if he had already bought George's present, James replied: "I haven't got that far yet."
James is the youngest brother of Kate and her sister Pippa Middleton, and the three are part of a close-knit family.
Asked how often he gets to see his nephew, James said: "We're a close family and we see each other and speak on the phone all the time, so the odd Sunday lunch or popping in for a coffee."
He also discussed the wedding in 2011 which was watched by two billion people, revealing that "the most wonderful thing about the wedding, which will stay with me forever, is that even though millions were watching, it felt like a family wedding"
James also revealed some surprising news — that Kate and WIlliam's pet dog Lupo was a gift from James. "My little treat for them was to give them a puppy and Lupo was their gift."
With another niece or nephew on the way, James said Kate's pregnancy was "wonderful" news, and he also admitted that he doesn't mind sharing the excitement with royal watchers, adding of Prince George's birth: "Everyone was celebrating with us, becoming an uncle, and for my parents becoming grandparents, they were all so excited."
The businessman was on air to promote his new sweet brand, Boomf, which allows people to print personalised photos or messages on to marshmallows. His partner Andy Bell revealed that James's official title at the company was "Wonka-In-Chief."