Children are using the journey to and from school to fill up on "a secret mountain of junk food", despite a drive to improve pupils' diets, research found this week.
It showed that nearly one in three primary children stop off to buy fizzy drinks, crisps and chocolate on the way to school, or while travelling home at the end of the day.
More than half of secondary pupils are spending extra cash on treats which amount to the equivalent of 20 blocks of butter and 11 bags of sugar over a year.
The research came from the School Food Trust, which was set up by the Government to drive improvements to school meals in England.
Judy Hargadon, the trust's chief executive, said: "Parents always want what's best for their children. But this survey shows that despite their good intentions, children are consuming a secret mountain of junk food on the way to and from school.
"Too often this is happening because the meal they are getting at school is simply not filling them up and the hungry child is topping up with sugary and fatty foods. The introduction of higher quality school meals from September will mean that parents can feel confident that their child can eat better and do better."
There are estimated to be two million overweight and 700,000 obese children in the UK.
The British Medical Association estimates that by 2020, one in five boys and one in three girls will be obese.
The issue of poor quality school dinners shot to the top of the national political agenda when Jamie Oliver, the TV chef, launched a campaign for improvements last year.
He spent time working in a school in east London for a series on Channel 4 which highlighted the poor quality of ingredients served to children in school canteens.
Ministers announced a pound;220 million package of reforms, including extra money to revamp kitchens and buy better ingredients.
New rules being introduced next month will require schools to ban all confectionery, such as chocolate, unhealthy savoury snacks like crisps, and fizzy drinks.
Manufactured meat products, such as chicken nuggets, will be restricted and must meet the standards for minimum meat content, the School Food Trust said.
Under the new rules, meals will have to include at least two portions of fruit and vegetables per day, as well as oily fish at least once every three weeks and healthier drinks like fruit juice and semi-skimmed milk.
The polling company BMRB questioned 412 parents with children in primary or secondary schools for the survey.