Luxury brands are looking forward to a multi-billion raise in sales thanks to the ‘Kate Middleton’ effect a new study has revealed.

Premium fashion brands such as Louis Vitton, Chanel, and Dior are expected to enjoy the strongest growth in the clothing sector, increasing 29 percent over the next three years.

The boost is estimated to be worth £8.6 billion (US$13.3 billion) by 2014, according to the report by Barclays Corporate.

The “Kate Middleton effect” has already increased sales at retailers Reiss and Whistles after Prince William’s wife to be was pictured wearing items from their collections.

The £650 Burberry trench coat she wore to announce her engagement sold out within one day.

While Kate’s fondness for the high street stores Reiss, Jigsaw, and Topshop has been well reported she often plumps for luxury British designers such as Temperley, Mulberry, and Burberry, meaning her choices have been a good blend of patriotic and classic. As she becomes more confident of her style it is the luxury brands that will prevail in her wardrobe.

Already, her former staple style of knee-high suede boots, cashmere cardigans, and knee-length skirts have been replaced with sophisticated nipped-in jackets, silhouette-hugging dresses, and timeless accessories that are elegant and regal.

Luxury brands are already well aware of what Kate can do for their sales.

In January Kate attended a wedding wearing a velvet black Dulwich coat, £310, by British label Libelula. Within hours of the picture being published, the coat had sold out and the company had thousands of hits on its website from around the world, with interest from the U.S. particularly high.

When she revisited St Andrews with William last month Kate wore a pillar-box red skirt suit (Sushi jacket, £335, and skirt, £160) by Italian designer Luisa Spagnoli.

In doing so she caused a stampede at the Knightsbridge boutique Hollie de Keyser, where the late Princess Diana also used to shop, which stocks the label. The outfit sold out within a day, with Hollie quoted as saying: ‘We have re-ordered 100 suits — that’s £50,000 worth of stock. Since Kate wore the suit, we have been inundated.’

Unlike celebrity endorsements, such as the infamous £500,000 Cheryl Cole was paid to be the face of L’Oreal, brands that associate with Kate tap into her role as a high profile ambassador. Members of the Royal Family do not accept paid endorsements.

According to the report the wider clothing sector is expected to report a nine percent “real growth” and the value fashion market is expected to grow 21 percent.

These figures represent a change in consumer behaviors: the trend now is to buy fewer, higher value items. In addition an older age demographic of shoppers who care about fashion place more emphasis on quality and service than price.

Retailers are expected to adapt their price structures and tailor their services towards older consumers to maintain growth and boost margins.

As Vogue editor, Alexandra Shulman, says: ‘When you get a pretty, young woman whose going to have an incredibly high profile on the stage the inclination is to want to embrace her.’