Nine out of 10 children's lunchboxes contain foods high in fat, salt or sugar, according to experts.
Most children do not have fruit in their lunchboxes
The Food Standards Agency surveyed over 550 children from across the UK.
It found children's packed lunches contained up to twice the recommended amount of sugar, half of their suggested daily salt intake, plus high levels of saturated fats.
The FSA says most packed lunches would not meet the nutritional standards set for school meals.
School meals, eaten by fewer than half of children, must offer at least one portion of fruit and one portion of vegetables, a portion of milk or a dairy item, a portion of meat, fish or other protein source and a portion of a starchy food, such as bread, pasta or rice.
In contrast, the most popular items in the children's lunchboxes were a white bread sandwich - found in 87% of packed lunches, followed by crisps
(71%), a biscuit or chocolate bar (60%) and dairy items such as yoghurts or fromage frais, found in 48% of packed lunches.
Fewer than half the children surveyed had a portion of fruit in their lunchbox.
The survey, which involved children from 24 primary schools across the UK at the end of April, found that 80% of those who took a packed lunch to school tended to have similar things to eat every day.
The FSA found up to 40% of saturated fats the children were eating came from butter and other fat spreads, up to 25% from cheddar cheese, up to 19% from crisps and up to 14% from chocolate bars and biscuits.
Salt tended to come from white bread, ham and crisps and the higher levels of sugar came mainly from fizzy drinks, ready-to-drink juice drinks and chocolate-covered bars and biscuits.
The FSA has published advice on how to create a nutritionally balanced packed lunch menus on its website.
Robert Rees, a chef and member of the FSA's board, said: "Parents face a daily challenge trying to get their children to eat healthy foods, and usually it's the children who call the shots when it comes to deciding what should go in their lunchboxes.
"Small changes to what children eat now can have a big impact on their diet and health in the future."
He added: "Healthy options needn't be boring, and these tips and lunchbox suggestions should be popular not only with parents, but also with the children eating them."
Data from the survey will be used in the Department of Health's 'Food in Schools' initiative, aimed at improving the nutritional content of packed lunches.