So could you fit into the new Size O (yes, zero) skinny jeans?

Zero. It may sound an implausible size for women to aspire to while squeezing into the latest fashions. No longer. The era of battling to claim that you are a size 8 rather than a 10 is so last year. Britain's high streets are downsizing at an unprecedented rate in a bid to make women feel good about themselves. A burgeoning number of British stores are clamouring to provide clothes in a stupefying US size 0 - a British 4.

And when US brand Abercrombie & Fitch pitch up in the UK later this year, skinny women will be able to snap up a pair of their favourite size 00 clothes - that's a British size 2.

The retailer TopShop has already started stocking size noughts in branches across the country, following in the footsteps of American chain Gap.

London's fashionable department store Harvey Nichols has bought in a range of designer labels in a zero, the French women's clothes shop Morgan now does a taille 0 and Australia's Wheels & Doll Baby is selling skin-tight dresses for the 0 frame in stores here.

The internet has also seen an explosion in sites selling 0 and 00 garments to British buyers. The move towards skinny clothes has been fuelled by some women's desire to mimic their celebrity icons and drop several dress sizes to squeeze into this season's drainpipe jeans.

Desperate Housewives' Eva Longoria and Teri Hatcher, 19-year-old model Theodora Richards, daughter of Rolling Stone Keith, Renée Zellweger and permanently thin US Vogue editor Anna Wintour all boast a svelte size 0 physique. Ally McBeal star Calista Flockhart and Posh Spice qualify for the ultra-petite 00 category, although they would never admit it.

The British Dietetic Association has sounded against the perils of dieting brought on by the new sizes. Spokeswoman Amanda Johnson said: "We need to remember that models such as Kate Moss are genetically blessed and that most people won't be able to attain their slimness. Crash dieting isn't a good idea; it can cause infertility, loss of bone strength and anaemia."

Yet, curiously, a recent UK national sizing survey showed the average woman is growing. Waists have gone from around 27in to a meaty 34in in 50 years and midriffs have swollen by nearly seven inches. Thighs are now more than an inch thicker and the average weight has shot up by half a stone.

But rather than have customers face up to the fact that women are bigger than ever, manufacturers are flattering them by vanity sizing -slapping smaller labels on larger clothes.

So a size 10 in Marks & Spencer or Next is quite different from the equivalent in TopShop or Miss Selfridge, which caters for a younger market. Harvey Nichols admits its size 0 labels, such as Marc Jacob, are equivalent to a slightly more generous British size 4 to 6.

One personal shopping assistant at the Knightsbridge store said: "Jeans by Marc Jacob, Moschino, Sass and Bide and Superfine are tiny, tiny tiny, but not a zero tiny. American zeros are funny sizes. You have to try them on before deciding."

TopShop's megastores in London, Birmingham, Manchester, Liverpool, Norwich, Glasgow, Liverpool and Dublin have been trialling size zeros in their petite range, in an experiment it hopes to make permanent practice. A spokeswoman for the retailer said: "We have been selling a small selection of size 0s in a range of tops, T-shirts and trousers to meet demand as part of a trial. We were getting lots of requests by teenage girls who were not quite a size 4 or 6 and wanted more fashionable clothes in smaller sizes. It started off being aimed at the young girl looking for something more grown up but it seems to have caught on among older women."

Tighter denim, made popular by model Kate Moss and actress Sienna Miller, has been blamed for contributing to an alarming rise in women crash-dieting.

Sass and Bide and Superfine, makers of skin-tight jeans favoured by Kate Moss, say shrinking is exactly what fashionistas are doing. A spokeswoman said: "We can't manufacture jeans in UK sizes 4 and 6 quickly enough. The average customer has dropped a size this season and is getting smaller. It's extraordinary - a size 6 is tiny if you're 5ft 6in plus.

"You can't get away with skinny jeans if you're carrying even an ounce of extra bottom or thigh fat - they're so unforgiving."

Size Matters: Have you got a WASP waist?

IF the American label says 00, you are either a seemingly impossible 21-inch waist - a British size 2 - or, more probably, ill.

Squeeze into a US 0, then count yourself a sophisticated, but still frighteningly thin British size 4. The likes of Jemima Khan is said to have worn such skinny jeans .

A US size 2 is equivalent to a British 6, which Madonna says she fits, while a more curvaceous 4 (that's a UK 8) counts among its fans Renee Zellweger on a thin day.

American 6s are our 10s - for women with 25-inch waists - while their 8s measure up to our 12s.

Tens, 12 and American 14s (add four to get the UK equivalent) and as far as Hollywood is concerned you might as well check into a fat camp opt for "one size fits all".

The Fit-Up: Hurrah! I can get into size 8 jeans

On a normal day I'm a size 12. On a very good day I'm a 10. So imagine my surprise and excitement when I find I fit neatly into size 8 standard cut jeans.

Only all is not quite as it seems I've been caught out by the stealthy Americanisation of our size structure.

When in Gap, where the shift in shape began, I feel thinner than I have in years. Ok, so I might not be able to squeeze into the size 0, but an 8 puts a smile on my face.

Elsewhere, it becomes clear the trend is spreading. With Top Shop and others following suit I can get into seemingly "smaller" sizes up and down the high street.

Even in the once-predictable Marks and Spencer, the size 12s sag around the crotch and make me look faintly ridiculous.

Hermione Eyre