The Atkins diet has helped to boost the sales of cheese and meat and led millions to fill up on steak for breakfast but devotees have now been warned they might be doing it all wrong.

Eating fat does not, after all, make you thin, as was previously assumed by many followersof the low-carb, high-fat diet, including the two million in Britain.

Experts say the diet has been misunderstood and that consumption of saturated fat and red meat should be restricted to lose weight.

The diet remains a high-fat eating plan, but the emphasis should be placed on the benefits of unsaturated fats that come from most vegetable oils and fish.

Colette Heimowitz, director of research and education for Atkins Nutritionals, has said that only 20 per cent of a dieter's calorific intake should come from saturated fat.

An Atkins spokeswoman added that Ms Heimowitz's message, which was delivered to health professionals in lectures across the United States, was not a departure from the founding principles of the diet.

Ms Heimowitz said she was highlighting the diet's limits of saturated fat because of some adverse comment from the medical profession. "We want physicians to feel comfortable with this diet, and we want people who are going to their physicians with this diet to feel comfortable," she said.

Robert Atkins, who created the diet 30 years ago, said that dieters should rely on a balance of different types of natural fat, and not just on red meat. But the publicity that surrounds the diet has stressed the free consumption of eggs, meat and cheese. The diet is widely understood to allow people to eat unlimited quantities of meat.

Ms Heimowitz said a limitation on fat would not work as "trying to tell consumers to do math is futile". She added that the Atkins books, which detail the diet and have become global bestsellers, state that dieters should "eat until you are satisfied but not stuffed".

But the diet remains a high-fat regime and 60 per cent of daily calories are derived from fat.

It is based on the theory that the body digests carbohydrates to form glucose, which is then converted to energy. Insulin controls the way in which cells take up the glucose.

If there is excess glucose, the body stores it as fat. According to Atkins, if you take carbohydrates out of your diet, the body begins to burn its fat stores.