Data released at a medical meeting in Canada shows that the drug produces a 4kg loss in the first four weeks and that over 80% of patients lose at least 5% of their body weight in the first three months.

This amount of loss has been shown to clinically improve patients' health and well being.

It also motivates patients to stay on their weight loss programme to make long term changes.

A second presentation showed Xenical helps maintain weight loss for up to two years.

Clinically obese

Professor Toplak said: "This study shows the significant benefits of rapid and continuous weight loss in the first 12 weeks of treatment with Xenical.

"Not only are there associated health advantages, but patients may also improve their motivation to lose weight and experience greater satisfaction with treatment."

"One of the biggest problems with weight loss programmes is that patients drop out because they do not experience early improvements in body shape or well-being."

Long term

A second study led by Professor Aila Rissanen, of the Helsinki University Hospital, Finland found that patients who took Xenical lost on average 10.4kg over a two year period.

These patients also recorded a drop in blood pressure and cholesterol levels.

The National Institute for Clinical Excellence decided earlier this year to make Xenical available on the NHS to people with serious weight problems.

It is estimated the drug will cost the NHS �12m a year.

Xenical, the brand name for the drug orlistat, works by blocking the digestion of fat. It is the first treatment that does not rely on suppressing the appetite.

More than 300 million adults world-wide are overweight, and most suffer from weight-related illnesses.

In Europe, up to 20% of men and 25% of women are classified as obese, with a Body Mass Index of 30 or more. BMI is calculated by dividing weight in kilograms by the square of your height in metres.

Treating people who are overweight accounts for up to 6% of total health care costs in developed countries.

The research was presented at a meeting of the North American Association for the Study of Obesity in Quebec City.