Ever since the Duchess of Windsor declared you can never be too rich or too thin, the diet industry has been making money out of women's neuroses about their shape and size. Critics estimate the business to be worth pounds 1bn a year with products ranging from diet foods and meal replacements sold by reputable retailers to outlandish, anti-fat capsules which promise to devour unwanted flesh overnight.
In her demand for regulation, Mrs Mahon (9st) makes little distinction between these. To her, all are charlatans who at best dent the pockets and self- esteem of millions of women and at worst encourage eating disorders such as anorexia.
'It is a dream industry,' Mrs Mahon says; she became concerned two years ago when two girls in her constituency became anorexic. 'You blame yourself when a product does not work and then move on to the next in the endless array.'
Mrs Mahon's bill would force slimming centres and products to display warnings against rapid weight loss. Books, tapes and videos would have to point out that permanent weight loss is unlikely and not guaranteed. Diets, slimming pills, potions and patches would be brought under the Medicines Acts.
Diet Breakers, co-ordinator of International No Diet Day, claims that 80 per cent of slimmers are women, that 90 per cent of females diet at some time and 50 per cent, including girls as young as eight, are presently dieting.
According to Professor Tom Sanders, chair of nutrition at King's College, London, slimming may yield short-term results but 96 per cent of people regain weight within three years because they have been drawn into a cycle of diet and binge. Only by fundamentally changing eating habits and gradual weight loss can this cycle be avoided. .
The slimming industry insists new regulations are unnecessary. Tessa Prior, spokeswoman for the diet food manufacturers - which account for about 10 per cent of the diet industry - claims that manufacturers' studies prove many customers do achieve permanent weight loss. She insists there is no link between slimming and eating disorders, which primarily affect women.
'The growth in criticism of diet products is paralleled by the rise of feminism. But the notion that dieting is a method of dominating women is silly,' she says.
There is plenty at stake. According to a survey by the analysts Mintel International, profits in the sector are rising again following a dramatic decline in the popularity of low- calorie programmes in the late 1980s.
Meal replacement products underpin this. The market leaders - Sun Nutritional (Slim- Fast), Nestle (Slender) and Boots (NutraSlim and Shapers) - have a turnover of pounds 88m a year.
Professor Sanders is exasperated by the refusal to accept natural body shape. 'In Britain food has replaced sex as the national neurosis. The EC is attempting to bring in legislation to keep this sort of obsession at bay. Marilyn Monroe was a size 16 and that is the average size today.'
As many as half of British women may not fit the size 10- 12 'ideal'. Comedian Dawn French has called her clothes shop for larger women 1647 because she believes 47 per cent of women are size 16 or over.
Dr Bridget Dolan, a psychiatrist at London's St Georges Clinic for eating disorders, says women particularly are under extraordinary pressure to diet in this country.
'A woman is judged first on her body's appearance. Self esteem is therefore linked to body image in a way it isn't for men.'