Women were asked to put "thigh-reducing" cream on one leg and an ingredient-free cream on the other.

After six weeks, there was no difference at all between any of the legs.

Chief researcher Dr Bonita Marks, assistant professor of physical education, exercise and sports science at the University of North Carolina said: "Manufacturers of these creams rake in the bucks for a product that doesn't work."

"Not only did we find no difference in each volunteer's thighs at the end, the volunteers themselves said they could see and feel no difference either."

The creams tested contained "aminophylline", and it is unclear whether any creams on sale in the UK contain the ingredient.

A spokeswoman for the cosmetic industry claimed that no creams containing it are sold in the UK, but at least two products on sale here, tested by a consumer magazine last year, say they contain "theophylline complex."

Hard to break down

The news will come as a disappointment to women who are keen to find a way to break down cellulite, a fatty deposit in the skin which seems remarkably resistant to every method of tackling it.

The excitement about aminophylline stemmed from laboratory research which showed it dissolving fat cells.

Both aminophylline and theophylline were originally used to treat asthma, as they act to relax the bronchial passages in the lungs which become inflamed and narrow during an attack.

Previous research which appeared to show that the ingredient worked in a cream was conducted on women who dieted and exercised during the study.

Diet or exercise changes results

The new study involved no dieting or exercise - and no cellulite reduction either.