WeightWatchers, the flabby older sister of the Atkins diet, is bursting at the seams. Four decades after it first unveiled its points plan, the world's best-known diet club is claiming its highest UK membership ever with more than seven million Britons - about one in six adults - now on the scheme.

The new figures reflect both Britain's growing obsession with diets and growing levels of obesity among adults and children. A survey by Datamonitor, a market research company, now estimates the UK diet market is worth a staggering £11bn.

But WeightWatchers' resurgence is also being put down to a combination of anxieties about the potential health risks of "crash diets" such as the Atkins, and the endorsement of celebrities ranging from Hollywood stars to the Duchess of York. A company spokeswoman said that the company's public profile had been hugely enhanced by the television programme Sex and the City, in which Miranda, as a new mother in search of her old figure, followed its point plan.

The dieting boom is also reflected in a substantial rise in business for two of Britain's other most popular diet regimes, SlimFast and Rosemary Conley. New research shows the number of UK dieters has more than doubled in the past 17 years. According to figures from the British Nutritional Association, one in three adults say they have dieted this year, compared to just one in six in 1986.

But WeightWatchers has admitted to having drawn up an internal strategy to combat the phenomenal popularity of Atkins, the high-protein, low-carbohydrate crash diet currently favoured by up to three million Britons.

While she declined to give any details, a spokeswoman for WeightWatchers confirmed that the company had been examining the impact of the Atkins diet in drawing up its more recent strategies. She said: "We are monitoring, as we monitor all our competitors, but there isn't any change planned in our philosophy or marketing strategy."

Though Atkins can count among its fans the likes of Catherine Zeta Jones, Brad Pitt and Geri Halliwell, the new research suggests that by far the nation's favourite dietary regime remains the WeightWatchers point plan. In the first six months of this year, 7.1 million UK adults were enlisted on its courses compared to 6.6 million in the same period in 2002.

WeightWatchers counts among its fans Hollywood stars including Drew Barrymore, Alicia Silverstone and Kirstie Allie, and the Duchess of York, who is a figurehead for the organisation. The duchess has, according to reports, helped to increase sales by up to a third.

Revealing the new take-up figures, Clive Brothers, WeightWatchers' chief operating officer, said: "We encourage people to eat a healthy, balanced diet, as we always have.

"If people want to lose weight longer-term, it doesn't help them to get into a 'yo-yo' situation where they keep going up and down," he said. "The problem with a lot of fad diets is that that's exactly what happens."

It is not just Atkins and WeightWatchers that have benefited from the huge surge in interest in dieting among British adults. SlimFast has reported a 20 per cent increase in take-up in the past year. Meanwhile, Rosemary Conley's low-fat, exercise-based diet plan can now claim over 30 per cent more followers than it had 12 months ago.

The heavyweight diets

By Annabel Fallon


What is it? Dieters are allowed 20 points per day. Which means a Big Mac and a gin and tonic at lunch allows for only a sandwich at supper. Weekly meetings - at £4.75 each - provide motivation. Joining fee of about £50 contributes to company's annual turnover of £250m.

Who does it? About seven million in the UK, plus 47 million worldwide. Since the now svelte Duchess of York became its spokeswoman, attendance has risen 60 per cent. Celebrity devotees include Drew Barrymore and Alicia Silverstone. White House followers include Vice-President Dick Cheney and the first lady, Laura Bush.

How does it weigh up? It seems to work. Look at Fergie, once dubbed the "Duchess of Pork". The points system is impressive but basically adds up to: eat less, weigh less.

The Atkins diet

What is it? The low-carb/high-fat diet devised by Dr Robert Atkins 30 years ago. Dieters are told to eat bacon and eggs, steaks and hamburgers - without the bun. In the induction phase, any type of carbohydrate is a no-no, as are alcohol, fruit and veg.

Who does it? About three million UK devotees, and an estimated 30 per cent of Americans have been on the diet or plan to be. Its high-profile followers include Jennifer Aniston and Minnie Driver. The Atkins bible costs £20.

How does it weigh up? It achieves rapid weight loss but involves sacrifices - curries without rice can wear thin. Unpleasant side-effects include bad breath and potential kidney problems.

Rosemary Conley 'Eat Yourself Slim'

What is it? Ms Conley, a grande dame of dieting, favours the traditional approach. Low-fat and calorie-light meals are combined with a healthy lifestyle in which exercise is mandatory. Dieters are also encouraged to attend weekly group meetings.

Who does it? Since her famous hip-and-thigh diet was introduced in 1988, Ms Conley's combined sales of exercise videos and diet books have totalled more than 8 million. Her bi-monthly magazine has a readership of 300,000. Her clubs are franchised, and have 90,000 members.

How does it weigh up? The diet claims to achieve steady, permanent weight loss. But be prepared to burn off those calories in the gym. You can't just rely on small portions to do the trick.

The SlimFast plan

What is it? Two shakes replace breakfast and lunch. Evening meals permitted. One SlimFast snack bar per day allowed.

Who does it? Robbie Williams and the designer Linda Barker are reported to be fans. According to Mintel, a market research firm, 2.3 million households in the UK used SlimFast products last year. Starter packs cost £1.99. Unilever paid £1.5bn for the company in 2000.

How does it weigh up? With so little to eat, the diet achieves quick weight loss. But critics argue that it does little to promote healthy eating. More of a quick-fix approach that will leave you gasping for a cooked breakfast.