This latest study, which examined 275 different cereal products, used the Food Standards Agency's (FSA) traffic light labelling system to test the amounts of salt, sugar and saturated fat in each cereal.
"With so much public concern about obesity and diet-related disease we're particularly concerned that most cereals marketed to children are still high in sugar, and many are high in salt too," said Sue Davies, chief policy director for Which?
With the latest EU figures estimating that 14m children in Europe are overweight or obese, and a 4 per cent rise in childhood obesity rates between 1995 and 2003 in UK children alone, the report is likely to add to the pressure placed on the food industry by scientists and consumer groups, who are increasingly calling for healthier product formulations.
The new report rates products green in areas where the FSA guidelines deem them to be healthier amounts per 100g, and amber to red where less so.
Most notable of these was the fact that only thirteen cereals received a green rating in terms of sugar content. In terms of salt, 'healthier' options such as Kellogg's All Bran, and Morrison's Right Balance, were given red labels, although these generally fared better when it came to saturated fat, with only 7 per cent receiving red lights. One of the cereals that did fall foul of the test "had roughly around the same amount of fat as a McDonalds McBacon Roll," said the report.
The latest findings echo those of a report conducted last month by the UK's Trading Standards Institute (TSI), which found that meals, snacks, and sweets aimed at children contained more than 20 per cent fat in some instances, while one product in particular contained 33 per cent.
Kellogg, which had some of its products tested for the recent report, responded in a statement that "research issued by Which? and the FSA Traffic Light colours are based on 100 gram portions. No cereal has a recommended serving size of 100 grams most are 30-45 grams."
The company also added that though it did not subscribe to the FSA's traffic light system, it agreed that consumers had the right to know what was in their cereal bowls, and could do so with the Guidance Daily Amounts included on the front of all its packaging.
The report was not all bad news for breakfast fans, however, with products like Nestle's shredded wheat, and Quaker's puffed wheat getting green ratings across the board.
While the Which? report also accepted that levels of salt had been slightly reduced since its last study of cereals two years ago, it clearly indicated that producers still have a long way to go in reducing sugars and fats before they will be receiving the green light from them in the future.