Action on Sugar said current guidelines for parents are baffling after they found many children's juices contain at least six teaspoons of sugar - more than Coke.
Official advice currently says a 150ml glass of unsweetened fruit juice counts towards five-a-day fruit and vegetables. Other juice drinks, such as squash and sweetened juice, do not.
A smoothie containing all the edible pulped fruit or vegetable may count as more than one five-a-day portion, but this depends on the quantity of fruits or vegetables or juice used, as well as how the smoothie has been made.
Action on Sugar nutritionist Kawther Hashem said: "Parents do not always understand the difference between a juice drink and a fruit juice. And most cartons come in 200ml or more.
"Many parents are still buying fruit juices and juice drinks for their children thinking they are choosing healthy products; children should be given as little juice as possible."
She said juice should be an occasional treat, not an everyday drink.
Chairman of Action on Sugar, Professor Graham MacGregor of the Wolfson Institute, Queen Mary University of London said: "It is a complete scandal that these drinks are marketed to children and parents as if they are 'healthy': this has to stop. We need to stop Britain's childhood obesity epidemic spiralling out of control."
Cardiologist Dr Aseem Malhotra added: "It is not just tooth decay but there is increasing scientific evidence that regular sugary drink consumption is associated with an increased risk of type 2 diabetes and heart disease, independent of body weight, suggesting we are all vulnerable."
Sugars are released from fruit when it is juiced or blended, and these sugars can cause damage to teeth and contribute to weight gain.
Added sugars shouldn't make up more than 10 per cent of the energy (calorie intake) you get from food and drink each day - or about 12 teaspoons - according to government guidelines. But it's not clear how much children should have.
Action on Sugar says the guidelines are baffling but Public Health England says the current advice is sound and that consuming five or more portions a day helps reduce the risk of heart disease and some cancers.
The World Health Organisation recently issued guidelines suggesting that cutting the amount of sugar we eat from the current recommended limit of 10 per cent of daily energy intake to 5 per cent would be beneficial.
The British Soft Drinks Association (BSDA) said fruit juice consumption in the UK equated to an average of just 45ml per person per day - accounting for 1 per cent of the calories in the average British diet.
BSDA director-general Gavin Partington said: "Given government figures show that the vast majority of adults and children are not getting their recommended five fruit and veg a day, it is unfortunate this survey omits to mention the established health benefits of fruit juice, such as vitamin C."
Dr Alison Tedstone, of Public Health England, said: "Fruit juice is a useful contribution towards our five a day, however, because the process of juicing releases sugars from the fruit we recommend that you try to limit your fruit juice to 150ml a day, including that from smoothies and only consume these and other sugary drinks with meals to reduce the risk of tooth decay."
More on Parentdish: Is 'healthy' fruit juice rotting your children's teeth?
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