A "large" number of parents are not including fruit and vegetables in their children's school packed lunches, it is claimed.
The World Cancer Research Fund (WCRF) has said 40% of primary school children's lunchboxes do not contain any fruits or vegetables, compared to 10% of school canteen meals.
The charity argues it is a "missed opportunity" for children's health. It is calling for a similar initiative to that spearheaded by celebrity chef Jamie Oliver for school dinner, urging parents to ensure their children's packed lunches always contain at least two portions of fruit and vegetables.
The School Food Trust urges parents to choose school meals over packed lunches.
The organisation estimates parents could spend almost eight days a year in the kitchen if they want to make packed lunches that meet the same nutritional standards as school meals.
"Our research has found that packed lunches are higher in saturated fat, sugar and salt than school meals," said Patricia Mucavele, a nutritionist at the School Food Trust.
"There is no doubt Jamie Oliver helped achieve great things for the food served in school canteens. But as the nutritional content of school canteen meals has improved, the healthiness of the content of lunchboxes has been left behind," said Kate Mendoza, Head of Education at WCRF.
"It is disappointing that children are going to school with lunchboxes that are not playing their part in helping to encourage the kind of healthy diet that is so important for their future."
"It can sometimes be difficult for parents to control what their children eat, particularly if they are passing shops on the way home from school or visiting their friends. But parents can influence what is in their packed lunches and the fact that not all of them are doing so is a missed opportunity."
WCRF cites a recent Ofsted report that claims parents are 'tired' of being told what not to put in their children's lunchboxes. Anecdotal evidence also cited by the charity is said to show schools are 'reluctant' to advise parents on packed lunches for fear of 'patronising parents'.
'My Packed Lunch' is an online game created by the WCRF to educate parents and children on how to include healthy foods "at affordable prices".