Health-conscious consumers should think twice before swallowing the claims made by makers of products containing "friendly" bacteria, experts said last night.

Up to half the "probiotic" products on the market are ineffective and some may even be harmful, they warned.

Friendly bacteria aid digestion in the gut and reduce the chances of having stomach upsets.

Research suggests they help prevent bowel conditions such as ulcerative colitis, protect children against allergies, and may even reduce the risk of colon cancer.

Recently there has been an explosion of probiotics in supermarkets, health stores and on the internet.

Products may be in the form of yoghurt-style drinks, supplements or powders. All claim to improve health by boosting levels of beneficial bacteria in the gut.

But scientists warned that many do not live up to the promises made on their labels.

They may not contain the type of numbers of bacteria advertised, and the microbes may not survive in the gut long enough to be of any benefit.

In some cases elevated levels of harmful bacteria have been found in probiotic products, it is claimed.

Consumers were also warned not to trust brands of "healthy" yoghurt promoted as being probiotic without any specific information about the bacteria they contain.

Professor Glen Gibson, an expert in food microbiology at the University of Reading, said: "As a rule of thumb, you can trust the big manufacturers. Their quality control is very good.

"On the other hand there are a lot of products out there that no-one's ever heard of, and this is where the problems arise. Half the products on sale don't match up to what they say on their labels."

Some 50 different probiotic products are currently marketed in the UK.

Prof Gibson's warning did not apply to major brands found in supermarkets, such as Yakult, Actimel and Vitality. But a plethora of other products are sold on the inter-net and in health food stores with no rules regulating what they contain.

"There's no legislation," said Prof Gibson. "You could buy a yoghurt-maker from Tesco, make your own probiotics, and sell them."

Experts believe high fat, low fibre western diets may contribute to a lack of friendly bacteria and an increase in harmful bacteria.

About 90 per cent of the bacteria in the gut of a newborn infant are friendly microbes, but this is reduced to 10-15 per cent in the average adult.

After 60-65 levels of friendly bacteria plummet 1000-fold.