Health experts are warning that the emaciated "size 0" look favoured by Hollywood stars is catching on with girls in Britain, desperate to emulate their idols.

The look - which equates to a UK size 4 - is encapsulated by what has been dubbed the "size 0 club", a group of Los Angeles celebrities whose high priestesses include Nicole Ritchie, daughter of the singer Lionel, and the Mean Girls star Lindsay Lohan.

Now the minute size, which can be found in stores across Los Angeles, is cropping up on the UK high street in branches of Topshop and Miss Selfridge.

It represents a shift in beauty ideals in which the "perfect 10" that used to represent the ideal female shape in Britain has been replaced by the tiny LA look. Natural waifs such as Keira Knightley show off ultra-thin figures on the red carpet, while Victoria Beckham, a mother-of-three, was recently reported to have worn jeans that would fit a seven-year-old.

Commenting on the trend for young women to aspire to size 0, David Haslam, clinical director of the National Obesity Forum and a GP, said: "I think it is irresponsible to promote this. It is not healthy for the individual. Fashions come and go, the pendulum swings to and fro, and this is clearly a case of it swinging too far. This is laying the ground for eating disorders in schoolgirls who see Kate Moss and aspire to her proportions. It is extremely dangerous."

The typical measurements for a size 0 are a 23.5in waist, 31.5in bust and hips at 34in. But experts warn that to achieve this body shape, a typical body mass index (weight/height ratio) would have to be lower than the healthy average of 25 to 29.9.

A very low BMI can lead to irregular menstruation cycles for women, leaving the chances of conception severely hampered, according to the British Dietetics Association (BDA).

The former Cosmopolitan editor Marcelle D'Argy Smith said the size 0 craze represented the "death of the grown-up woman". When she was editing Cosmopolitan, she added, there was a ban on articles on dieting and advertisements for cosmetic surgery. "Speaking as an older woman, I think it's absolutely ghastly. I went to the launch of Diane Von Fustenburg's jewellery collection recently which had the cream of upmarket gossip fodder, and I saw people like Liz Hurley and Jemima Khan who I thought were absolutely minute."

D'Argy Smith believes those who portray size 0 as a "normal" body shape are perpetuating a cult of thinness. "What is so bizarre about all this is that they are being praised for their bodies. Who is defining these bodies? Some gay men in the fashion industry who want to hang their clothes on stick insects," she said.

Rachel Cooke, a public health dietitian, from the BDA, said the size 0 fad was capturing the imaginations of celebrities at a time when both the United States and Britain were attempting to tackle obesity issues.

"Two out of three adults are overweight or obese and at the other end of the scale, we have celebrities aiming to be a size 0 - a sizing that is now being highlighted in the UK and that young people will soon be made very aware of and may start trying to achieve," she said.

While celebrities get their ultra-thin look with the help of stylists, nutritionists and personal trainers, teenage girls achieve the look by rash dieting and exercise "binges", dietitians fear.

Ms Cooke said: "Many of the high-profile celebrities seem to be obsessive about either dietary intake, activity levels or both. For those trying to emulate such celebrities, the advice would be to forget fad diets and excessive activity regimes. Instead, think of your long-term health and happiness. By incorporating a healthy varied diet that includes your favourite foods in moderation and by taking part in activity that you enjoy, a healthy weight becomes a pleasure and not a chore. Enjoy your meals and activity sessions rather than dreading the next fad diet- inspired meal or boot camp-style activity session."

Karen Fenn, buying director for Topshop, said that its size 4 was intended for girls aged between 12 and 14. Only 2 per cent of sales in the Petite range were size 4. Ms Fenn said: "It's for a small and young customer. It's not something that is by any means for a grown woman."

Miss Selfridge was unavailable for comment.