A British health group is warning parents against packing their children's lunchboxes with fruit juices, saying some contain more sugar than Coca-Cola.

Well-meaning parents who would not dream of giving their children Coca-Cola might instead be packing their lunchboxes with juices that are even more sugary, a health group has warned.

British group Action on Sugar surveyed more than 200 juices, smoothies and fruit drinks to find that more than a quarter contained the same level of sugar or more as Coca-Cola, which has 10.6g for every 100ml.

Action on Sugar nutritionist Kawther Hashem said it was a concerning finding, given rates of childhood obesity and tooth decay.

She said the survey looked specifically at juices that were aimed at children or marketed as lunchbox-friendly.

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.

Copyright AAP 2014.