Many of us assume fruit juice is a healthy way to start the day - after all, in the last year we've been advised to up our fruit and veg intake from five a day to seven by several experts.
But a survey from Action on Sugar of 200 of our favourite juices, smoothies and fruit drinks has revealed more than a quarter contain the same level of sugar as Coca Cola - which has 10.6g for every 100ml - or more.
The survey looked specifically at juices that were aimed at children or marketed as lunchbox-friendly.
Among the worst offenders identified by the Queen Mary University of London-based campaign group were Asda's Chosen by Kids Tropical Juice From Concentrate, which contains 13g of sugar per 100ml, and Tesco Goodness Slurper Apple & Banana Fruit Smoothie Snack for Kids, which contains 16.1g of sugar for 100ml.
Official advice from the NHS currently says a 150ml glass of unsweetened fruit juice counts towards your five a day.
Action on Sugar nutritionist Kawther Hashem said the new research make this advice concerning, given rates of childhood obesity and tooth decay.
Fresh fruit juices - which cannot contain additives like extra sugar - tended to do better than fruit drinks or juices made from concentrate, she said.
But she warned parents against seeing the word 'juice' on a label as a green light.
"It wasn't clear-cut, but I do think the ones at the top of the sugar list are usually from concentrate," she said.
She said parents were better off giving their children diluted juice or - better still - water and a piece of fruit.
She also recommended parents steer clear of artificially sweetened drinks, "because you're still encouraging the sweet consumption ... These are young children. You're training their taste buds - we would prefer that you give that child a piece of orange than something that tastes like orange".
Action on Sugar has called for manufacturers to reduce the amount of sugar they add to their products, and for the Government to withdraw its advice that a small glass of unsweetened fruit juice can count towards fruit and vegetable intake recommendations.
Commenting on the study, Dr Sally, NHS weight-loss surgeon & founder of vavista.com
"Don’t be lulled into the marketing ploy of an ‘all-natural’ juice – sugar is sugar, and we need to keep on top of our consumption."
But 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% of the calories in the average British diet.
"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," BSDA Director-General Gavin Partington said.
"Then again, one should not be surprised that politically motivated campaigners are prepared to ignore the evidence in pursuit of their goal."