Designers say the newest royal has the power to boost retail sales fivefold simply by wearing one of their frocks.
Top British designer Alice Temperley told the BBC that Middleton is "breathing new life" into royal clothing and "looks fantastic."
"She's supporting British fashion by wearing British fashion, which is brilliant," said Temperley, who, along with other big-name designers, such as Victoria Beckham, Stella McCartney and McQueen, are all showing collections at the 2012 event.
Jane Shepherdson, the head of U.K. fashion retailer Whistles, once underestimated the Duchess Effect after Middleton selected one of its ivory blouses for her engagement photo to Prince William.
"I don't think Kate's made a difference to sales, maybe a few hundred pounds. A pretty dress is a pretty dress; it's going to sell, regardless," Shepherdson said at the time.
Shepherdson was singing a decidedly different tune at London Fashion Week a couple of days ago.
"We're about to start our international expansion," she told the London Telegraph. "We've had so much publicity with the duchess wearing Whistles. We've got to do it now rather than wait too long. We've just opened in Russia and we're expanding to the U.S. and Southeast Asia."
Reiss saw a similar impact on its sales last year when Middleton stepped out in one of its $340 camel-colored dresses to greet President Obama and the first lady last May. Traffic on the Reiss website rose by 200 percent, Reiss said, and the dress quickly sold out.
The family-owned, British retailer's operating profits rose from around $6.6 million to $13.2 million, according to filings at Companies House in the U.K.
The Duchess of Cambridge is not only setting trends, she's following them too. Crowned Hat Person of the Year 2011, Middleton has commissioned outrageous milliner Piers Atkinson to make her hat for the launch of the queen's Diamond Jubilee tour. Atkinson has designed head-topping creations for pop divas Lady Gaga and Rihanna