Almost half the products assessed contained a child’s entire daily recommended maximum sugar intake of 19g (almost five teaspoons).
Smoothies were the worst offenders, with 13g of sugar per 100ml.
"Well-meaning parents have been misled by the recommendation that a glass of juice or a smoothie should count toward their child's 'five-a-day'.
Edelstein continued: "In fact, the high levels of fructose in fruit juice and smoothies have the same effect on blood sugar levels as the sugar in a fizzy drink, and are contributing to the current child obesity epidemic."
The findings were published in the online journal BMJ Open ahead of the publication of the government’s childhood obesity strategy.
To assess the sugar content of fruit juice drinks, 100% natural juices and smoothies marketed specifically to children,
Researchers measured the quantity of ‘free’ sugars per 100ml in 203 standard portion sizes (200 ml) of UK-branded and supermarket own label fruit juice drinks, 100% natural juices and smoothies, which are marketed specifically to children.
‘Free’ sugars refer to sugars, such as glucose, fructose, sucrose and table sugar, which are added by the manufacturer, the BMJ stated.
They are not the naturally occurring sugars found in whole fruits and vegetables, which the body metabolises differently and which act to curb energy intake.
The results highlighted wide variations in the amount of free sugars between different types of drink and within the same type of product.
The sugar content ranged from 0-16g per 100ml, with the average measurement being: 7g per 100ml. But sugar content was significantly higher in pure fruit juices and smoothies.
Among the 158 fruit juice drinks analysed, the average sugar content was 5.6g per 100 ml, but rose to 10.7g among the 21 pure fruit juices tested.
This again rose to 13g per 100 ml among the 24 smoothies assessed.
More than 40% of the total sample of products (85 juice drinks) contained at least 19g of free sugars - a child’s entire daily maximum recommended amount.
"Fruit drinks and smoothies are popular choices with young children and they do contribute to nutrition by providing vitamin C and other sources of vitamins and minerals," Charlotte Stirling-Reed, child nutritionist at SR Nutrition told The Huffington Post UK.
"The trouble with drinking fruits, rather than eating them, is that you get much less in the way of fibre and the sugars are more readily available to get stuck around tiny teeth.
"This is part of the reason why the Government’s new ‘Eatwell Guide’ allows 100% juice and smoothie options to contribute to no more than one portion of your '5 A Day'.
"It’s also why the Government recommends that fruit juices and smoothies should be limited to a total of 150ml a day."
Almost 60% of all the products in the BMJ study would get a "red traffic light label", which is a coding system designed by the UK’s Food Standards Agency to help consumers identify high levels of fat, salt, and sugar in processed food and drink.
The BMJ also noticed that in line with European law the labels on all the products contained reference intakes, (guidelines based on the approximate amount of nutrients and energy you need for a healthy, balanced diet).
However, this figure applies to an average sized adult woman who does an average amount of physical activity, and so are "wholly inappropriate for children", the researchers emphasised.
Stirling-Reed offered some tips for parents wanting to give their children juices.
"Choose small cartons of 150ml or less, offer juices and smoothies alongside meals, such as at breakfast," she said.
"Also, dilute fruit juices with 50% water, choose vegetable juices over fruit juices, and, ultimately, make sure that children get most of their fruits and vegetables from whole fruits and veggies, and don’t rely on drinking them."
Edelstein added: "Like other sugary food and drinks, juice and smoothies are best reserved for the occasional treat and children should stick to water as their default drink."
The BMJ made five recommendations based on their findings:
1. Fruit juices/juice drinks/smoothies with a high free sugar content should not count as one of the UK government’s ‘5 a day’ recommendations, as is currently the case.
2. Fruit should preferably be eaten whole, not as juice.
3. Parents should dilute fruit juice with water or opt for unsweetened juices, and only allow these drinks during meals.
4. Portion sizes should be limited to 150ml a day.
5. Manufacturers should stop adding unnecessary amounts of sugars, and therefore calories, to their fruit drink/juice/smoothie products—and if they can’t do this voluntarily, the government should step in with statutory regulations.