Fruit juices, smoothies and fruit drinks marketed to children and their parents contain “unacceptably high” levels of sugar, often as much in one small carton or bottle as a child should consume in a day, according to research.

The paper published in the journal BMJ Open comes in the wake of the chancellor George Osborne’s announcement of a tax on sugary soft drinks – but fruit juices and smoothies are exempt. The researchers say tough action is needed to reduce the amount of sugar children consume in fruit drinks that are bought and sold on the assumption that they are healthy.

“These are marketed intensively to children as well as to parents,” said Prof Simon Capewell of the department of public health and policy at the University of Liverpool, one of the authors. “They are routinely packaged in garish colours. They routinely have cartoons and other sort of folksy animal creatures being used to market them.

“There is often a health halo – some claim about vitamin C or ‘packed full of fruit’. There are no restrictions around the words industry can use in their marketing. They can claim or imply quite a lot. Then we end up with more than a third of these drinks having more sugar in them than a cola or fizzy drink.

“I think it came as quite a surprise to us really that there is so much sugar hidden and that any of the most familiar brands had such a high level.”

One in five children aged four to five years and one in three children aged 11 are overweight or obese. Children aged between four and 10 get 30% of their sugar intake from soft drinks. Sugar is also responsible for decaying teeth, which is the most common reason for children in England to be admitted to hospital.

The researchers analysed 203 fruit juices, fruit drinks and smoothies stocked by seven major supermarkets – Tesco, Asda, Sainsbury’s, Marks and Spencer, Waitrose, the Co-op and Morrisons. They found almost half contained a child’s entire recommended daily intake of sugar, which is a maximum of 19g or nearly five teaspoons.

They looked for added sugar or naturally occurring sugars in the juices and smoothies. The average sugar content was 7g per 100ml, but in fruit juices and smoothies, it was significantly higher. Among the 21 fruit juices analysed, it averaged 10.7 per 100ml and among the 24 smoothies, it averaged 13g per 100ml.

Almost 60% of the drinks would get a red traffic light label on the basis of their sugar content, the paper says. The researchers say that 78% of the drinks contained a non-calorific sweetener such as aspartame. Although these have been declared safe by the European Food Safety Authority, they write, they believe a reduction in the overall sweetness of drinks is necessary to reduce children’s desire for sugary things.

Eating fruit does not pose the same problems as drinking the juice, say the authors of the paper. “One key difference between whole fruit and juice is fibre content,” they write. “Whole fruit slows down consumption and has a satiating effect. Research shows the body metabolises fruit juice in a different way compared to whole fruit.” Drinking juice and smoothies does not seem to reduce children’s appetite in the way that eating fruit can do.

The revelations of the high sugar levels in fruit juices and smoothies will dismay parents who may have turned to what they see as a healthier option than sugar-sweetened fizzy drinks such as colas and lemonades.

The researchers recommend that fruit juices, fruit drinks and smoothies with high sugar content should not count as one of the “Five a Day” recommended by the government. “Ideally, fruit should be consumed in its whole form, not as juice,” they write. “Parents should dilute fruit juice with water, opt for unsweetened juices and only give them during meals. Portions should be limited to 150ml a day.”

The campaigning group Action on Sugar called for government action to require industry to reformulate soft drinks and cut the sugar content by 50% within the next five years. It also wants children’s sugary drinks to be sold only in 150ml sizes. UK guidelines say fruit juices and smoothies can count as one of the five fruits and vegetables children should eat a day but only in 150ml servings. The study found only six products that came in 150ml cartons.

Gavin Partington, the British Soft Drinks Association’s director general, said: “It is completely misleading to suggest that 100% fruit juice and fruit juice smoothies contain added sugar, they are not allowed to by law and contain only naturally occurring sugars from the fruit. At the same time they provide essential vitamins and nutrients which many people in the UK today are sadly lacking.

“Only last week Public Health England confirmed that 150ml of fruit juice or fruit juice smoothies can contribute to the five a day target. Very few people reach their five a day target and given the positive contribution it has to the diet, it is counterintuitive to suggest that 100% pure juice should not contribute to it.”