Cereals targeted at slimmers often have no fewer calories and different ways of measuring fibre make comparisons almost impossible, the guide, published by the Consumers' Association, said. In 1994, consumers in the United Kingdom ate pounds 1bn worth of cereal.
Perfect Balance, produced by Weight Watchers, contains only 2 calories per 100g fewer than Kellogg's Bran Flakes (318 cals per 100g compared with 320 cals per 100g), while having 25-30 per cent less fibre. And, despite advertisements that show sylph-like bodies, Kellogg's Special K has the same amount of calories and double the sugar of Kellogg's Cornflakes.
Added sugar was a problem in many cereals. An unsweetened cereal contains less than 1g of sugar per 100g. But added sugar in Sugar Puffs makes up nearly half its total weight.
It is illegal to make false claims that products can help prevent, cure or treat a disease but the Consumers' Association says some cereals imply they will be good for the heart. Bran Hearts, from Jordans, come in a heart shape and Kellogg's Common Sense is pictured in a heart-shaped bowl on the box.
Which? also expressed concern about cereals aimed at children. A recent survey looked at television advertisements appearing at children's peak viewing times. It found that almost one in five of the food and drink advertisements were for cereals and three-quarters of these cereals were high in sugar.
Brands such as Kellogg's Coco Pops, Frosties and Corn Pops, billed as "part of your nutritious breakfast", contain more than 40 per cent sugar.
Kellogg's yesterday defended its advertising of children's cereals, saying that they contained many nutrients vital to health and growth.