The dress is not the first thing you notice, of course. It's the woman wearing the dress. Beautiful, elegant, a certain 'screw-you' insouciance. Kate Moss. And she demands to be looked at, really looked at.

That's the thing with Kate Moss. That's why she's become the world's most ubiquitous model, selling us everything from make-up to handbags to ready-to-wear. You stop and look.

In a fashion market where brands spend more than £700m on advertising - and that's without the £500m that gets spent on cosmetics and toiletries, often by the same fashion houses in the same glossy magazines - standing out from the crowd is a marketing must.

So you notice Kate first. Then you notice the dress. It, too, is beautiful. A striking blue, lovely halter collar. And this time it's Kate's dress. No, not one she's been 'lent' by a famous designer in the hope she'll wear it out in front of the paparazzi. Kate's dress, designed by Kate. She chose the shape, the colour, the fabric. Now it could be your dress, if you're quick. It is launching in selected Topshop stores on May 1, part of the new Kate Moss Topshop collection.

Topshop is betting millions of pounds and its street cred that the dress and the rest of her designs (stripy shorts and waistcoats, cocktail dresses with a twist, satiny masculine suits, floaty girly dresses) will have us slavering to get the Moss 'look'.

Welcome to the world of celebrity designers. This is Moss as fashion collaborator. Somewhere within the DNA of the Kate Moss collection is the spirit of Kate herself, apparently. Buy the denim hot pants, buy a piece of Kate.

And Topshop is quick to stress the level of attention paid by their star turn to the clothes that bear her name.

She is just the latest in a growing line of celebrities who are now becoming 'brand architects' - extending their own brands, and coming off the covers to get into bed with serious retail businesses. Madonna's just done it in 32 countries with H&M, Lily Allen is launching Lily Loves clothes in 312 stores across the UK, northern Europe and the Middle East on May 9.

Except that we're not fooled. Moss, Madge and Lily - sure, they know plenty about clothes, all right. Mostly the sort of clothes few of us can afford. But does anyone really think they can do a better job of designing our high-street fashion than the retailers' own designers, the guys who have spent years studying their craft and their market?

Of course not. This is not about supplementary designer talent; this is about a shrewd, multi-layered marketing strategy that works seamlessly from initial product concept through to advertising, PR, and the bottom line. Not surprisingly in this cut-throat market, the retailers themselves are cagey about exactly how the deals are cut. H&M insists this is not just about licensing the Madonna name and face; this is about partnership.

"Madonna had a key role in the collaboration," says an H&M spokeswoman. She says they chose Madonna because "we share the same view of fashion, a desire to express ourselves in a unique and individual way ".

Except that there's actually nothing unique and individual about the M collection. This is not celeb designer-fashion-as-PR-stunt - all hype and little stock. This is nationwide distribution of so much clobber that you'll see your friends in it. So there hasn't been the sort of gold rush that H& M saw when it launched its limited collection with Stella McCartney last year; which is great news if you've got your eye on one of Madge's trenchcoats.

Moss's line for Topshop will be a little more exclusive, there are just one or two hundred copies of some of the pieces, but the 91- item collection will be updated through the seasons, so there's always reason to go back in store to check out what's new.

Kudos

Neil Mason, a senior retail analyst at Mintel, believes that introducing a limited collection, like Kate Moss's at Topshop, adds to the appeal. " The celebrity's status in itself is enough to make these lines work, but there's no doubt that when a retailer limits the supply it generates more interest, a real kudos for the people who manage to get their hands on a coveted item."

So it's no surprise that many of the shoppers scouring the rails for the more elusive pieces from the Kate Moss collection will not be wearing the clothes themselves. Within hours they'll be up on eBay, the subject of a frenzied bidding war.

With this sort of interest from shoppers, it's not hard to see why these retail tie-ups can be lucrative deals for the celebrities themselves. Green is said to have paid £3m to sign up Moss, but as the two are poised to take the collaboration worldwide, it's likely Moss will also get a share of the profits.

For shops like Topshop, H&M and New Look, celebrities are a crucial element of the marketing mix. Fashion is snappy, disposable, quick, quick, quick. These days, the fierce competition on the high street means stock has to move through at pace. Which is a marketing challenge. There's little time to build brand values around a range, to nurture an image; that has to be done instantaneously but also consistently. Look at Marks & Spencer's campaign starring Twiggy; big name, famous face and they're pumping out lots of ads, quick turn-around. The models are the glue that holds the different campaigns together. Madonna, Allen and Moss will do the same for their retailers - they're all brands that provide a sort of shorthand, a rich seam of positive credentials that other brands can piggyback. Instantly recognisable, Madonna equals funky, healthy, confident, in charge. Lily Allen is cool, now, outspoken, individualistic. And Moss is the epitome of personal, accessible style: streetwise, edgy, a little bit dangerous, always beautiful.

But, you might wonder, are they worth the money? Madonna is a decade or two older than the teen-to-twenties H&M heartland; she's a mum; she's, well ? not quite the hot musico she once was. But the fact remains that she's ubiquitous. The clothes, the ad campaign, the PR - all will work around the world wherever H&M wants to take it. That's cost-effective marketing for you.

Global brand

The same with Moss. She's a risk. When those infamous grainy pictures appeared and she was dubbed Cocaine Kate, several brands that had signed her up as their 'face' ran scared and pulled the plug. But history has proved the Moss brand to be a resilient one; she reportedly made more than £30m in the year following the scandal.

And so far Topshop's boss, Philip Green, appears to have made a shrewd move with Moss. She is, after all, a global brand, famous in plenty of places that Topshop isn't. As he begins laying plans for his US invasion, taking the Topshop proposition into America's retail heartland, it's no surprise that Moss will be there at his side and her collection will be the draw that gets the fashion pack moving.

Talking of danger, there's no cast-iron guarantee, of course, that these affiliations will fly. In fact, the more of them there are, the more the novelty wears off. You see it in the perfume industry, where every celeb from Sarah Jessica Parker to Jade Goody has launched their own smell. Most of them tend not to linger. Even when the brands seem like a great fit, there's no guarantee.

Big bucks

Of course, retailers pay big bucks for such associations: Moss' £3m-plus, Madonna's rumoured six-figure deal plus a share of the action. But consider that a top notch 'face' like Moss or Liz Hurley can command fees topping £1m simply to star in a single ad, and the sums start to make sense. Uma Thurman's just pocketed a reported £18m to promote Virgin Media.

The logic behind the budgets makes sense if you flick through any glossy mag: you'll find almost as many pages of ads as you will editorial.

But celebrities in ads are nothing new. Getting your celebrity to infect your product, to become part of your product, your brand DNA, now that's a different game.

The rise of the celebrity mega-brand is in some ways a logical follow-on from the longer-standing, cosy relationship between stars and fashion PRs. A good PR company with a range of fashion clients will have a department dedicated to winning celebrity endorsement for their brands.

The luxury freebie might seem like a pointless exercise when you consider it is the privilege of those who can actually afford to buy the products in the first place. But for the chance of a full page in the Daily Mail or the fashion mag society pages, it's an invaluable tool in the marketing armoury, and these "endorsement" campaigns are executed with military precision.

It was only a matter of time before the recipients of such goodies, seeing how valuable their endorsement could be, would want to cut out the middlemen and launch their own ranges.

And let's not forget that these are celebrities that themselves thrive on publicity. All this exposure is pretty good for the Madge, Moss and Allen brands and, assuming the fashion collections are a success (odds on), rocket fuels their next venture. In the case of an ageing pop star or a 32-year-old model, it also offers the opportunity to extend their careers and their earning potential just that little bit longer ...

Claire Beale is the editor of Campaign magazine