Where Kate Moss goes, the high street follows - and a brand-new way of vintage dressing is born.

I'm looking at a rail of clothes at Oasis HQ in East London. Hanging next to a Sixties floral chiffon dress with a shimmering sequin collar is its version for summer 2004. The original was picked up for Pounds 200 at the Clignancourt flea market in Paris. Beneath its two pretty pink and yellow layers, the hot pink lining is cut into a tight, sexy pencil skirt. The Oasis one is almost identical. "But ours is more floaty," explains Nadia Jones, the design director of Oasis. "It has three layers, no underskirt and the cut is more generous around the neck and arm holes. Women didn't mind being trussed up then. They won't stand for it now."

The dress comes from a range called New Vintage, which, as its oxymoronic name suggests, is a small collection of one-off vintage finds from various eras, reproduced for the mass market at higher than normal prices. "We know our girl likes the idea of vintage because she sees Hollywood stars and Kate Moss wearing it," says Jones. "But she either doesn't know where to get it, or can't be bothered to search for it. So we do it for her in her dress size with no holes or stains." When it launched last October, New Vintage was a first on the British high street, and also an immediate success. Topshop soon followed, with a line of recreated vintage clothes chosen by Bay Garnett - a contributing fashion editor to British Vogue and newly relaunched thrift fanzine Cheap Date - and remade, only to sell out in two days flat.

That the high street is openly cashing in on the cult for old clothes, and the public are lapping it up, shouldn't come as a surprise. The trend for genuine vintage clothing has spawned a spin-off cult for hunting down unique fashion treasures, and whether the hunt takes place in Oasis or Portobello Market doesn't seem to matter.

Each of this season's most trend-setting collections are inspired by old clothes. Marc Jacobs is Thirties. Prada is Fifties. Chloe is Seventies. Nike, adidas and Levi's reissue old styles as a matter of course. Even Patrick Cox is on a vintage trip. "Feet grow by a quarter-size every generation," says Cox, "so very few women fit authentic vintage shoes." For spring, Cox is using dusty old-fashioned colours, shapes from the Thirties and aged leather to make brand-new shoes look like they were discovered in an antique market. "Everyone likes the look of old shoes, but who wants to wait for a shoe to age?"

Given that new vintage is currency, what exactly is genuine vintage these days? "People use the word 'vintage', but they don't have a clue what it means," laughs Gerry Richards, who has manned the floor at his store, Cornucopia, in London since 1968. Real vintage, he says, is anything pre-Second World War. Unofficially, though, most dealers apply the term to clothing - preferably with a designer label - made before the mid-Eighties.

Recognised, and often deceased, designers who summed up their era, such as Ossie Clark, Bill Gibb, Lanvin or Thea Porter, are collectible now. And prices go up if the vintage clothes parallel current fashion. Ossie Clark dresses went for Pounds 250 before his V&A retrospective (which ends May 2), now they go for Pounds 400-Pounds 600.

Beaded flapper dresses are more expensive this spring because they're in fashion. Clothing with innovative workmanship, such as that found in early Westwood, Galliano and Zandra Rhodes, is also collectible. When Westwood's V&A retrospective opens on April 1, prices for her early 1975-1989 work will go though the roof; and it's already happening. Platforms, like the ones Naomi Campbell fell over in, have doubled in price.

As the cult of vintage has grown, the people who sell it have become fashion pundits. Cameron Silver, owner of West Hollywood vintage boutique Decades, was listed as one of the most influential people in fashion by Time magazine in 2002, alongside Anna Wintour and John Galliano. But, tellingly, it is not Silver's movie-star clients Nicole Kidman and Renee Zellweger buying his old Chanel, Norell, Ossie Clarks and Hermes that make him money. It's the designers ("just about everyone you can think of") that use his clothes for inspiration who bring in 60 per cent of his business.

Mark and Cleo Butterfield of C20 Vintage Fashion closed their London retail operation 18 months ago and moved their vast archive to Devon. Their real business comes from paying clients Topshop and Oasis, several designers including Marc Jacobs, and Hollywood celebrity stylists. "The market has totally changed in the past five years," says Mark Butterfield. "Old-style vintage collectors loved how things were made, and bought accordingly," he says. "Our celebrity clients now buy one-off vintage items in the same way as women used to buy couture, because they want and often need to look fabulous and unique."

"Kate Moss pioneered the vintage trend," says Steven Phillips of West London store Rellik. "She was one of the first to realise she couldn't get what she wanted from current designers and brands, so she turned to vintage. And now look at it. The mass market is saturated with the idea.

You can buy 'vintage' in Brent Cross, for goodness sake." Phillips, 42, an Eddie Izzard lookalike, collects Westwood, early Galliano, shoe designer Terry de Havilland - an inspiration for Miu Miu - and Ossie Clark. His proudest moment, he says, is the day he sold Kate Moss a pair of Vivienne Westwood buckled pirate boots. "She was photographed in them, and suddenly everyone wanted a pair. Next thing, Shelley's are doing a buckled boot, and, four years later, they're still selling by the truckload."

So what's the future? Is the idea of being "in fashion" dead? "The woman I sell to now is beyond fashion. She is interested in terrific clothes regardless of trends," says Cameron Silver. "I liken it to having only ever listened to Britney Spears," says Madeline Meyerowitz, creator of St. Louis-based online boutique enokiworld.com. "Then someone hands you Exile on Main Street by the Rolling Stones and you realise there's something better." Personally speaking, I feel there is a time and a place for both.

-- Melanie Rickey