The image of cereal bars as a healthy snack is a "myth", according to a Which? study that found many contain high levels of fat and sugar.
All but one of the 30 bars in the study were high in sugar, with 16 containing more than 30% sugar, found the consumer group.
One bar, Nutri-Grain Elevenses, contained nearly four teaspoons of sugar (18g) - more than in a small 150ml can of cola (15.9g) and 20% of the recommended daily allowance.
The Tracker Roasted Nut bar was found to be almost a third fat, and while some of this came from the peanuts and hazelnuts that also provide some nutritional content, the ingredients also contained vegetable fat and harmful hydrogenated fats. Monster Puffs, a cereal bar marketed to children and described as "great for your lunchbox", contained 43.5% sugar, or more than two teaspoons, Which? found.
Six of the seven cereal bars targeted at children were 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 the healthiest choice for children with a low salt content and medium levels of fat and saturated fat, the watchdog concluded.
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.
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."