To the untrained eye, the Duchess of Cambridge was wearing a pleasant but nondescript simple blue and white hooped top to go sailing off New Zealand.
But the £48 garment has now sold out after royal fashion followers flocked to copy Kate's nautical look.
Maker Me+Em was inundated with buyers after she wore its "Breton top cobalt/white" while piloting an America's Cup sailing boat off Auckland during her tour Down Under with husband William.
The top is part of a look popular with the 32-year-old Duchess, who wore an almost identical outfit, right down to the skinny navy jeans and wedge shoes, when she attended a vollyball session at London's Olympic Park in October.
Lisa Haynes, the Press Association's fashion and beauty editor, said: "I'm not surprised that Kate Middleton's Me+Em top flew off the shelves. A Breton top is a real wardrobe summer staple and seeing the Duchess wearing one instantly gives the piece fresh appeal.
"It's not exactly experimental to wear a nautical look to go sailing but that's where the Duchess of Cambridge triumphs - she wears classic looks with poise and elegance. A Breton top like Kate's is never going to date or look tired after one season.
The top's page on the Me+Em website had a message today reading: "Sold out due to popular demand - more stock arriving soon."
It described the top as "a perennially chic style", featuring "flattering" features like a scoop neckline, three-quarter length sleeves and "ruched detailing to the bottom hem".