The drug was shown to help successful dieters keep weight off in a study published in the American Journal of Clinical Nutrition.

After losing eight per cent of their body weight over six months without the help of the drug, patients taking orlistat regained only around half the weight in the following six months compared to those carrying on without it.

In addition, the orlistat group showed a greater reduction in cholesterol than those not taking the drug.

However, AJCN Editor-in-Chief Charles Halstead said the results did not herald the arrival of a solution for obesity.

He wrote: "One must question why, after one year of treatment, three fourths of patients had begun the relentless process of weight regain."

Orlistat is marketed in the UK as Xenical. Manufacturers Roche Products believe it could be suitable for more than 15% of the UK population.

The drug works by blocking the absorption of fat in the gut and should be taken as part of a low-fat diet.

If people taking it eat a fatty diet, they experience a bloated and painful stomach and oily diarrhoea.

Patients given Xenical have been shown to lose an average of 10% in body weight over a year, compared with a 6% loss for those who only follow a low-fat diet.

Resource fears

More than 15 per cent of people could be suitable for the drug

The Royal College of Physicians issued tough guidance on the prescription of obesity drugs earlier this year.

Under RCP guidelines overweight patients will only be prescribed drugs such as Xenical as a last resort if diet and exercise changes have failed..

The guidelines stipulate that treatment should be stopped if the patient does not lose 5% of their weight within three months.

The guidance followed concerns that huge demand for Xenical would drain NHS resources.

Conversely, some doctors were also concerned that health authorities tended to view obesity as a self-inflicted problem and were reluctant to pay for its treatment.

The Obesity Resource Information Centre (ORIC) has warned that some patients lose more weight than others on Xenical.

The ORIC stressed that Xenical is not a miracle cure and should only be used as part of a weight management programme.

Obese people are defined as those whose weight in kilos divided by their height in metres squared is over 30. This measurement is known as the Body Mass Index.