In an analysis of 30 cereal bars from the best-selling brands, the consumer group found that all but one was high in sugar, with 16 containing more than 30% sugar.
Of the 30 bars analysed, 10 were high in saturated fat and six of these are marketed to children.
One bar, Nutri-Grain Elevenses, contained nearly four teaspoons of sugar.
The only bar that didn’t contain any added sugar was Planet Organic’s Nákd Apple Pie bar.
The report highlighted the fact that manufacturers don’t make it easy for consumers to see how much sugar has been added as there was so many different types of sugar in the same bar. Which researchers found 18 different forms of sugar in the same bar.
Which? executive director Richard Lloyd said: “People often choose cereal bars in the belief that they’re healthier than chocolate or biscuits, but our research shows this can be a myth.
“Manufacturers need to be clearer about how much sugar, fat and calories are loaded into each bar so that people can make an informed choice. We want all foods to have a traffic light colour-coding system so people can see easily what they’re eating and giving to their children.”