A number of popular breakfast cereals have been found to contain as much sugar as seven and a half chocolate fingers per portion.
Campaigners warned that such high levels contribute towards childhood obesity.
The study, carried out by campaign group Action On Sugar, also found that there has been an increase in the amount of sugar found in certain types of cereals over the past three years.
It found 14 out of 50 cereals were at least a third sugar – 33.3g per 100g, or eight teaspoons in each serving.
The worst offender was Aldi's Harvest Morn Choco Rice at 39g per 100g, which was up by 18 per cent since a survey by the consumer group Which? in 2012.
Morrisons Honey & Nut Corn Flakes and Sainsbury's Honey Nut Corn Flakes came in at 36.3g per 100g – up by 8 per cent compared with three years ago, although this is disputed. The amount in Kellogg's Frosties was 37g per 100g and that for its Crunchy Nut was 25g per 100g, neither of which has changed.
Experts claim many manufacturers are ignoring advice to cut sugar, which has been dubbed the new tobacco over fears that it could become as big a national health problem as cigarettes.
Action On Sugar nutritionist Kawther Hashem said: 'You wouldn't give your child chocolate biscuits for breakfast, yet certain manufacturers are effectively doing that for us.'
