The image of cereal bars as a healthy snack is a "myth", according to a study by consumer group Which?
It found 29 out of the 30 bars it analysed - some aimed at children - were high in sugar, with 16 containing more than 30%.
The watchdog said some of the products "should be on the sweet counter, not marketed as health foods".
Nutri-Grain Elevenses, has nearly four teaspoons of sugar (18g) - more than in a small 150ml can of cola (15.9g).
The Tracker Roasted Nut bar was almost a third fat, while Monster Puffs, a cereal bar described as "great for your lunchbox", contains 43.5% sugar.
Six of seven cereal bars targeted at children which were analysed were found to be high in saturated fat.
Which? compared the nutritional content of the bars using the manufacturers' information and applied 'traffic light' labelling to see if the levels of fat, saturated fat, sugar and salt were high, medium or low.
The Nakd Apple Pie was the only bar in the study that did not contain any added sugar, while the Alpen Light Apple and Sultana was the only one to have three green 'traffic lights' for fat, saturated fat and salt.
The Weetabix Oaty Strawberry Crusher bar was deemed the healthiest choice for children, with a low salt content and medium levels of fat and saturated fat, the watchdog concluded.
It is calling for manufacturers to reduce sugar and fat in food products marketed to children and for tighter controls over the way they are promoted.
Which? executive director Richard Lloyd said: "People often choose cereal bars in the belief they're healthier than chocolate or biscuits but our research shows this can be a myth.
"With high levels of sugar and saturated fat in some of these products they should be on the sweet counter, not marketed as health foods.
A spokeswoman for Kellogg's, which makes the Nutri-Grain Elevenses bar, said: "We're confused as to why anyone would call a Nutri-Grain Elevenses snack a cereal bar. If you've eaten one you know it's not.
"It's a baked bar and looks and eats much more like a muffin or cake. We bake it like a cake and market it as a mid morning snack. In fact compared to other similar mid morning snacks, it's one of the choices that has slightly less sugar than the norm."