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.
"Her casual look also has accessibility for Copy Kates. Unlike some of her formal Alexander McQueen or Jenny Packham looks on the royal tour, the Breton is something her fashion followers can add to their everyday wardrobe."
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".