This is not going to be prescribed for somebody who wants to get into a size 12, rather than a size 14. This is for people with serious obesity

Professor Ian Macdonald

But health experts say the drug will only be used for the very severely obese and not as a quick slim diet.

Last resort

The drug works by blocking the absorption of fat in the gut and should only 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 just a 6% loss for those who follow a low fat diet only.

About half the adult population is overweight or obese

Professor Ian Macdonald, Professor of Metabolic Physiology at Nottingham University, said the drug works alongside a healthier lifestyle.

He said: "It will only work to help reinforce people's attempts to stick to a low fat diet.

"People will have to have demonstrated some weight loss before they get the drug," he said.

Professor Macdonald said the drug would not be a quick fix for those wanting to lose a couple of pounds.

He said: "This is not going to be prescribed for somebody who wants to get into a size 12, rather than a size 14. This is for people with serious obesity."

Dr Wendy Doyle, spokesperson for the British Dietetic Association, said the drug will be vital for people who see no other way to get slim and have tried more traditional methods.

She said if properly prescribed the drug will benefit patients and the NHS, as cutting the weight of patients will boost health and lead to fewer people being hospitalised at even greater costs to the NHS.

"Orlistat is only used when most other routes have failed and for those people I believe that anything that will help will be welcomed.

"It will help alleviate the enormous health problems that you have with obesity."