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 has found.
Nearly one in three primary school 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.
And more than half of secondary school 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 ministers to drive improvements to school meals in England.
The trust's chief executive Judy Hargadon 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 starting in September will mean that parents can feel confident that their child can eat better and do better."
New rules being introduced next month will require schools to ban all confectionery, such as chocolate, unhealthy savoury snacks like crisps, and fizzy drinks.
There are estimated to be two million overweight and 700,000 obese children in the UK.
