When Catherine wore a £159 cream Reiss dress for her official engagement photos, it sold out at a rate of one every minute, crashing the company’s website in the process. Some customers put their business hats on and took advantage, selling their purchase for as much as £750 on ebay. The going rate for the dress today is still £380!

Its called the ‘Kate Effect’.

And Reiss in particular, as one of the Duchess’s preferred brands, seems to have benefited from it. Last week they reported that profits from the UK arm of the business doubled from £4.2 million to £8.5 million. And this is at a time when many retailers are feeling the effects of an economic downturn.

But its not just a UK phenomenon. Within 12 hours of wearing a Burberry trench coat at an official engagement in Belfast, it sold out across the globe.

Anything that the Duchess wears sells out across the globe. We explore the so called 'Kate Effect'.

The Duchess of Cambridge stole the show again at the War Horse Premier last week in a full length Alice Temperley lace dress. In many ways it was reminiscent of her wedding dress, which inspired many bridal dresses across the globe. And judging from comments made on Twitter, sales of this dress may go up too.....

BumpWearClaire:: 'I adore Kate Middleton's Alice Temperley dress worn to #warhorse premiere. I want.'

Unlike other celebrities who officially endorse a product, or who get sent piles of clothes to promote, the Duchess of Cambridge has no official relationship with any of the chains she buys from. David Reiss the founder of the fashion chain told the Evening Standard: ‘We never know when she might wear something. She's been a customer of ours for four or five years now and we've always been very understated about it.’

Retailers have always benefited from celebrity endorsements. Combining this with a royal seal of approval, seems to have the power to make people forget about their economic woes.