Xenical, the brand name for the drug orlistat, will be available in Britain from September after winning approval for EU regulatory authorities.

The drug works by blocking the digestion of fat in the gut. It has to be taken as part of a low-fat diet. People taking the pill on a high-fat diet will get a bloated and painful stomach and suffer oily diarrhoea.

Xenical is not suitable for pregnant women, anyone under 18 or anyone with gastric problems.

It is the first fat-blocking slimming drug to get the go ahead. Other slimming aids tend to work by suppressing the appetite.

Two - Adifax and Ponderax - were withdrawn from sale in the UK recently because they were associated with heart problems.

Xenical will be available through doctors for the clinically obese, particularly those with other complications like diabetes and hypertension. These are people whose weight in kilos divided by their height in metres squared is over 30.

Nutritionists are worried that it could become available on the black market because of people's desperation to find a quick cure to weight problems.

Trial

In a recent trial, orlistat has proved almost twice as effective in reducing weight as dieting and it halves the risk of piling the pounds back on when people return to a normal diet.

The Lancet reported that patients given orlistat lost an average 10% in body weight - around 25lbs - over a year, compared with a 6% loss for those who only followed a diet.

People on orlistat were half as likely to put weight back on than those on a placebo.

Xenical, marketed by Roche, is already on the market in the Far East and Latin America, but has yet to be approved in the USA, where obesity is at epidemic levels.

In the UK, obesity has doubled since 1980.

Exercise

Xenecal could give you stomach aches if you eat too much fat

Amanda Wynne, a nutrition scientist at the British Nutrition Foundation, says British people actually eat less than they used to, but they burn up less calories than they lose by not doing enough physical activity.

She is in favour of Xenical, but has some reservations. She says it has to be combined with a low-fat diet and an exercise regime to be effective.

"It is not a magic new wonder drug, It is not an alternative to diet and exercise," she said.

But she said it was worth trying if you were clinically obese. "It may be a bit of extra help. We have had 30 years of messages about exercise and diet and obesity has gone up dramatically and it is better than extremes like stomach stapling," she stated.

And she said the side effects associated with taking the drug on a high-fat diet could put people off guzzling on pizzas and other fatty foods.