Energy drinks should be banned for children under 16, the campaign and research group Action on Sugar says.
The team surveyed the nutritional labels of 197 drinks found in supermarkets and online.
One had up to 10 teaspoons of sugar per 250ml, twice as much as others surveyed. It is thought teenagers get 30% of their sugar from soft drinks.
Health officials say government campaigns already encourage people to have fewer sugary drinks.