The battle of the bulge is soon to get a drug-fuelled boost. The first over-the-counter weight loss pill has been licensed in the US and is expected to be approved for use in Britain next year.

Alli, marketed by GlaxoSmithKline, will join dozens of slimming aids sold in chemists and supermarkets. Unlike them, Alli has been proved to work. Clinical trials have shown that when combined with a low fat diet and exercise regime over six months, Alli can increase weight loss by 50 per cent, Jean Paul Garnier, the head of Glaxo, said.

In the US, the Food and Drug Administration gave the go-ahead to Alli last month, triggering a marketing campaign that the company hopes will reap five to six million US customers a year. Priced between $12 and $25 a week (£5-12), Alli could have annual sales of $1.5bn.

Glaxo said yesterday that it planned to apply for a European licence by the end of the year with the launch of the drug expected to follow within 12 months. Alli is a version of Xenical, the prescription weight loss remedy launched by the Swiss pharmaceutical company, Roche, in 1999.

Two thirds of adults are overweight or obese and the numbers are continuing to rise. Experts believe diet and discipline are not alone sufficient to prevent waistlines expanding and that extra help is required.

The drug works by blocking the absorption of fat from food that is eaten. About one quarter of the fat is blocked with prescription-strength Xenical, but the effect will be reduced in lower strength Alli.

This unabsorbed fat has to go somewhere and dieters using it are prone to diarrhoea, its least attractive side-effect. Steve Burton, the Glaxo executive in charge of promoting Alli, has used the drug himself and described to reporters the embarrassment it caused when he ate a fish and chip meal - the result was a dash home for a change of clothes. Still, Mr Burton, 48, is a walking advertisement for Alli. Over three years his weight fell from 275lb to 210lb.

Evidence from trials of Xenical also show that weight loss is temporary - and tends to be regained once the drug is stopped.

A spokeswoman for Glaxo in the UK said: “Alli is a fantastic drug if taken alongside a low fat diet and you have a lot of will power. But it is not a wonder pill and it won’t achieve weight loss on its own.”

Dr Colin Waine, the chair of the National Obesity Forum, said: “My concern is that it should not be seen as a panacea for people who want to get into a smaller-sized bikini. It may also be unpleasant - if you eat a high fat diet you will experience the effects. But if used sensibly with the right sort of dietary back-up then this could help some people.”

Independent News and Media Limited