London, England (CNN) -- An ordinary English girl in an extraordinary situation is prompting stylish women in London, New York and even further afield to emulate her relaxed, smart way of dressing. It's known as the "Kate Middleton effect."
The sapphire-colored Issa dress she wore at the announcement of her engagement to Prince William, heir to the UK throne; the white Reiss dress she wore in the engagement photos; and a simple monochrome Topshop dress she was papped in on her 25th birthday all sold out after she was seen wearing them.
It would seem that when Prince William's elegant bride-to-be wears something, women want to emulate her.
So, it's not surprising that all eyes were on what Kate was wearing Thursday as the couple marked their first official joint engagement in Anglesey in Wales. Or Friday when they returned to alma mater, St. Andrew's University in Scotland, where they met.
In Anglesey, Kate flew the flag for British clothing, wearing a pheasant feather hat by Vivien Sheriff and fashionable boots from upscale UK chain store Russell and Bromley.
It's this mix of designer and chain store clothing that makes Kate's style seem accessible and easier than most to imitate.
Such is the demand for her look that her designer $600 blue Issa dress sold out from British stores hours after she was photographed in it. And so did a bargain $25 copy of the dress sold online by UK supermarket Tesco.
Daniella Helayel, the Brazilian founder of Issa says the "Kate effect" has boosted her business unbelievably. "Not just the dress -- everything (sold out). It was amazing," she said at London Fashion Week.
The phenomenon is not just confined to the UK capital. It's being felt in New York too.
Bloomingdale's fashion director, Stephanie Solomon, says that Kate's ladylike but youthful style resonates with New York customers at the moment.
Kate "really epitomizes in some respects the all-American girl, that's sporty, outdoorsy, fresh-faced, with shiny hair, pretty dresses, comfortable footwear," she said.
So strong is the trend, hazards Solomon, that if Kate were to wear a dress stocked at one of Bloomingdale's North American department stores, it was sell out within hours.
That's exactly what happened with the Reiss "Nannette" dress. According to Laurie Marco, President of Reiss in North America, the understated white dress sold out within just two days of arriving in U.S. stores.
It's not just that women want to look like the girl who got Prince Charming, according to Reiss's Brand Director, Andy Rogers. "American girls like a clean look, with glossy hair," like Kate's he said, adding that Kate's manageably chic style was bound to chime with them.
Marco agrees: "She has an elegance and timeless style about her that the Americans admire. There is also so much excitement and energy around the wedding here in the U.S.."
"She defiantly reaches the people in terms of the simplistic style she has, she is stylish in affordable clothes and accessories," Marco continued.
Ultimately, Rogers thinks women relate to her because she has a very real way of expressing herself in her clothing, mixing designer labels with chain store brands in a manner not unlike Michelle Obama.
"People perceive people like Kate to have a limitless supply of designer clothes, they're surprised by it," he said. "But in general, this is what we all do, this is the real world, real people dress that way."
Stylish women all over the world will no doubt be keeping their eyes peeled for versions of what Kate has worn this week; shops had better prepare themselves for a stampede.