Counteracting the bars' image as a healthy breakfast choice, the group discovered high levels of fat and sugar in leading brands.
The survey follows market research from Leatherhead International last month showing that the cereal snacks industry was benefiting from consumers choosing what they believe are low fat, low sugar snack options.
But such marketing has been slammed by Which? who claim the bars' nutritious image disguises hidden fats and sugar - making them as unhealthy as cakes.
The group said: "Many people eat a cereal bar instead of breakfast; they're often promoted for their healthy credentials and many are targeted at children.
"But every one of the cereal bars was so sugary it would get a red light for sugar under the Food Standards Agency's 'traffic light' labeling scheme."
The group investigated the nutritional content of 20 top-selling cereal bars and discovered that all were high in sugar while more than half were high in saturated fat.
Cereal companies Kelloggs, Nestlé and Jordans were singled out in the report and criticised for the unhealthy content of their breakfast style bars.
A Kellogg's Fruit 'n Fibre bar contained 10g of sugar per bar while Jordans Original Crunchy Honey & Almond bar was the worse offender in terms of fat content with 6.8g per bar.
Jordans said in response that all its products were made with natural ingredients and to cut out fat would require including artificial additives. The company added that 87 per cent of the fat in the bar was 'good' fat necessary for health.
Leatherhead research shows that last year cereal bars took $2.37bn (€1.88bn) in snack sales in the US, Japan, Australia and Europe's five largest countries.
And growth seems set to continue with the researchers predicting a 2010 sales total of $3bn (€2.38bn).