She may be the Duchess of Cambridge but she's also the queen of Britain's shops.

The girl once called Kate Middleton is undeniably a fashion icon with huge influence – and now she's got cash tills ringing at retailers right across the United Kingdom.

Economists credit the 29-year-old with helping to boost consumer confidence figures, released in the UK Friday, while April retail sales also grew as women across the nation scrambled to get the Kate look. Everything she wears - seemingly everything - is scrutinized, analyzed and promptly sells out within hours. It’s a dream come true for the sales department but a nightmare in terms of I.T.

A case in point: The blue dress the Duchess wore to announce her engagement to Prince William helped to boost sales by 45 percent for the London label Issa.

This week, unprecedented demand for a flesh-toned satin number she donned to meet the U.S. President and First Lady crashed the website of its maker Reiss. The garment retails at just $300 – a pretty penny, certainly, but hardly an extravagance for one so wealthy. It was also reportedly several times cheaper than the designer outfit chosen by Michelle Obama, showing it’s hard to put a price on elegance.

The Middleton effect is worth big money, especially in a premium clothing market which Barclays Corporate estimates will be worth a third more by 2014 at just shy of $15B.

And no British fashion chain has benefited more from the Duchess’s patronage than Reiss. The company began with just one menswear store on the fringes of London’s financial district, known as The City. Today, it’s a multi billion-dollar business, with 106 stores around the world. Reiss now employs 1,500 people and has branches in 10 countries, including the burgeoning economies of China, Russia and the United Arab Emirates.

That’s good news because Middleton mania is a truly global phenomenon. The dress the Duchess wore this week is now completely sold out on both sides of the Atlantic. 

It’s not the first time Britain’s newest royal has provoked a surge in demand for its designs. On four separate occasions the Duchess turned up to events wearing Reiss, prompting huge interest in the brand and a fair bit of overtime for its press relations team. Reiss’s suitably hip Head of Media is Helena Choudhury. She says the Middleton effect has been “absolutely phenomenal” for the company.

“She is one of the most watched women in the world at the moment so anything she wears is bound to have an impact,” Choudhury says. “Of course it will have an impact on sales but it also introduces us to completely different type of customer.”

Kate may now be married but the Middleton boost lives on in the form of her younger sister Pippa, who after a stellar performance at the royal wedding is now making waves for her wardrobe too. A blue blazer she wore after the big day sent fashion bloggers into a frenzy. As you can imagine, that’s likely to be good news for its maker Zara, owned by Spain’s textile giant Inditex.

The Middleton sisters’ whimsical choice of clothes has helped change the way companies manufacture their garments. When Kate sported Reiss’s white “Lynette” dress to her official engagement photo shoot with legendary snapper Mario Testino, Reiss’s phone began to ring.  After releasing the dress for a second time in January, the store is now on it's second order. By the time I spot it in their flagship boutique, they are down to their last three (sadly none are my size).

Choudhury concedes it’s challenging reordering items at such short notice.  The dress in question, she says was unavailable at the time the pictures were released as it was from a previous season. “We had to re-release it and it sold out very quickly afterwards. It takes a few weeks to produce the clothes but luckily people are prepared to wait.”

The Duchess of Cambridge’s trendsetting and dedication to Main Street is one of a number of factors changing the way women consume all sorts of apparel across the UK.

In a new book called “To Die For” writer Lucy Siegle estimates that the average woman demands about four times as many clothes today as she did in 1980.  If that doesn’t make you think, listen to this: Because clothes have become cheaper, Siegle says the average female shopper in Britain accumulates about 28 kilograms of them each year.

That may be good for those who sell clothes - but it’s a nightmare for the environment with over 2,500 tonnes of clothing going to landfills each year across the country.

For those shoppers as keen on saving the planet as updating their image, the solution comes in the form of initiatives like The Clothes Exchange, set up by Marks and Spencer and Oxfam in 2008.  These schemes encourage customers of certain stores to give their old clothes to charity in return for discount vouchers.

Endorsing an initiative like that would win the Duchess of Cambridge more brownie points in my book than her love of modestly-priced dresses.