According to the researchers from University of Leeds, who conducted a major survey in UK, the lack of fresh food is to blame.
"This study underlines the role that parents, carers, the government and the food industry have in ensuring children eat more healthily," said study researcher Charlotte Evans from University of Leeds.
"The research has found that on some fronts, packed lunches have improved but they are still dominated by sweet and savoury snack food and sugary drinks. The vast majority provide poor nutritional quality. Addressing that issue over the next 10 years will require a concerted effort," Evans added.
For the findings, published in the journal BMJ Open, the research compared the nutritional quality of packed lunches brought into a sample of primary schools in 2006 and then in 2016.
The results reveal how the nutritional quality of lunchboxes has changed over 10 years.
It is estimated that more than half of primary schoolchildren take a packed lunch to school.
Over the 10-year period, the researchers found that many children did not have any dairy foods in their lunch, and meals did not meet the recommended standard for calcium.
There was a reduction in the number of packed lunches meeting the standards for vitamin A, vitamin C and zinc.
According to the study, there was no reduction in saturated fats. There was no reduction in the portion size of crisps.
The researchers said the food industry has not focused on reducing the size of savoury snacks in the same way it has on sweet snacks.
Although the amount of sugary food in lunchboxes declined over ten years it is still higher than recommended.
The researchers investigated whether packed lunches met the food standards that apply to cooked meals in England's schools.
Since 2006, eight standards have been introduced for cooked school lunches.
Confectionery, savoury snacks and sweetened drinks are restricted while vegetables, protein and dairy have to be included in each meal.
Packed lunches, however, are not subject to any control.
The researchers found that the percentage of packed lunches meeting all eight food standards was very small, increasing slightly from 1.1 per cent in 2006 to 1.6 per cent over ten years.
"Improving what children eat at school will help reduce the risk of childhood obesity," Evans said.
( With inputs from IANS )