The image of cereal bars as a healthy snack is a "myth", according to a study by Which?
The consumer group found all but one of the 30 bars it analysed were high in sugar, with more than half containing over 30% sugar.
One bar, Nutri-Grain Elevenses, contained nearly four teaspoons - more than in a small can of cola and 20% of the recommended daily allowance.
Other snacks it analysed were found to be high in fat and saturated fat.
The Tracker Roasted Nut bar, for example, was almost a third fat.
Meanwhile, six of the seven cereal bars targeted at children were high in saturated fat, the study found.
And Monster Puffs, a cereal bar marketed to children and described as "great for your lunchbox", contained 43.5% sugar - more than two teaspoons.
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.
Which? 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."
A spokeswoman for Kellogg's, which makes the Nutri-Grain Elevenses bar and some of the other snacks tested, 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."