"It is a collectors item, we commissioned only 500 and just have a few left. The mug has been sold all around the world," Lynda Tillotson, a Middleton family friend who runs L Interiors, told HELLO! Online.
"The mug came about because I had not produced anything here in my Interior Shop in Bucklebury for the wedding and we had visitors from all around the globe upon us and I had nothing to offer them," she added.
The £19.95 mug, which was specially designed by Doris & Co in collaboration with the Berkshire interiors shop, features a crowned acorn and the words "From Small Acorns" with the year 2013 etched alongside.
"The Avenue of Oaks is very important here in Bucklebury because it is so ancient, originally planted for Queen Elizabeth the 1st visit to Bucklebury," Lynda said. "Kate has spent most of her formative years living on The Avenue and as you can see has incorporated the Acorn into the family crest."
The news of the mug selling out comes in the wake of William and Kate releasing an adorable image of them with their three-month-old son on the day of his christening.
The personal photo shows the new parents looking down lovingly at their contented little Prince as he beams back at them.
Twitter was aflutter on Saturday evening as the fifth snapshot from the christening was released with royal watchers marvelling at the picture-perfect young family.
Since his birth on 22 July, the future King has become a true trendsetter despite the public only having seen photos of him on three occasions.
The bird-print wrap by aden + anais which he was wrapped in while in his car seat as he left the Lindo Wing at St Mary's Hospital flew off the shelves.
It wasn't just the cute wrap that sparked a shopping whirl — the little Prince's very first 'outfit' also benefitted from the endorsement.
Appearing on the steps of the Lindo Wing with his proud parents, the newborn was presented to the world in a white shawl by GH Hurt & Son in Nottingham, the same company that made William's blanket 31 years ago, and reportedly also that used for Prince Charles in 1948.
Shortly after the royal birth, company director Gillian Taylor admitted the firm were struggling to cope with the demand.