'Brands want to position their clothing, jewellery and accessories with beautiful celebrities,' says Dev Suj of Brandista, a consulting firm focusing on the luxury sector. 'The people who become consumers buy because they're inspired.'

A spokesperson for luxury brand Celine says that celebrities raise awareness of brands by wearing designer labels - and can also boost their status as being fashionable. 'They set trends and it's a mutually beneficial relationship.'

Why has the recent trend favoured celebrities over models? According to experts, it boils down to personality. Unlike models, who are often unknown outside of fashion circles, celebrities already have a public image, giving them a degree of intimacy with the public that models don't have.

'An endorser, celebrity or otherwise, gains credibility when consumers think they're likeable, when consumers think they're similar to themselves, and when consumers are familiar with them,' says Gerard Prendergast, professor of marketing at Hong Kong Baptist University. 'Celebrities are powerful in the sense of being likeable, and in the sense of consumers being familiar with them. Research shows that celebrities are effective when endorsing products that are purchased primarily for image rather than functional reasons.'

The benefits are twofold. 'It's no longer convincing just to portray a stunning model in advertising campaigns,' says the Celine spokesperson. 'The celebrity has to have a certain persona and talent that a brand wants to associate with and be able to project the brand's essence through their own style, personality and charisma.'

But the bottom line is to increase sales. Why else pay millions of dollars to secure a celebrity in the first place?

The trend of using celebrities in fashion marketing has reached fever pitch. Images of celebrities are plastered on billboards and even product labels. Recent print campaigns by Dunhill, Louis Vuitton, Christian Dior and Celine have featured Jude Law, Scarlett Johansson, Sharon Stone and Emmanuelle Seigner.

'Empirical evidence clearly shows that there's an image transfer process,' says Prendergast. 'The image of the celebrity is transferred to the image of the brand.'

Product placement is also growing in importance. A paparazzi shot of a celebrity toting the latest designer bag can trigger a buying frenzy. The term 'it bag' probably wouldn't exist if not for celebrity association. The Hermes Kelly and Birkin bags are each named for the women who carried them, actresses Grace Kelly and Jane Birkin. Anya Hindmarch's 'I'm not a plastic bag' bag is the latest in demand, since it was photographed with Keira Knightley and Lily Allen.

'Seeing a celebrity in her everyday environment is far more likely to encourage the consumer to want to acquire a product than the rarified occasion of dressing for the red carpet,' says Sue Evans, a retail trends forecaster and senior editor at Worth Global Style Network.

It's standard practice within the industry, including in Asia, to give free gifts to celebrities in the hope that they'll be photographed wearing or using the item. 'Media or paparazzi photographs offer quick awareness for a label or a product - and free media coverage,' says Annie Wong, executive creative director of advertising agency Ogilvy & Mather in Hong Kong.

Prendergast says there's 'considerable evidence of celebrity endorsement leading to an increase in sales, especially for products that have a large image component'.

So powerful is the lure of celebrity that high-street retailers have taken the trend one step further by collaborating with the celebrities themselves on clothing collections. 'High-street retailers are experiencing great sales and, most of all, publicity from collaborations with celebrities,' says Evans. 'The Kate Moss and Topshop tie-in has been hugely successful for the high-street brand as has the Kylie and Madonna collections at H&M.'

Celebrity endorsement is one of the main marketing strategies adopted by luxury labels in Asia. Celebrities are signed up as so-called spokespersons for brands, based on their marketability. A public relations executive at a high-end French luxury label in Hong Kong says 'the strategy is often to invite suitable celebrities to events, as opposed to using the more overt strategy of product placement'.

Prendergast says that Asian consumers 'tend to be more conspicuous and collectivistic than their western counterparts'.

'They often consider the display of famous brands to be an indicator of success and they like to follow the in-crowd. This suggests that there's a bigger role for celebrity endorsement in Asia.'