According to a group of dieticians, the average lunch box, which children get from home, contains far more saturated fat and sugar than a typical school dinner.
The health experts says it is parents who are putting children's health in jeopardy by adding "treats" like crisps and chocolate to their lunch boxes.
"Thousands of children will be heading towards strokes and heart attacks if they carry on eating like this, so it's very serious," the Daily Express quoted Dr Gail Rees, a dietician at Plymouth university, as saying.
"Crisps and chocolate are OK once in a while but on a frequent basis, they can be very damaging. Also, cheese, ham and bread, the ingredients of the most popular sandwiches, all contain high levels of salt and fat," the medical expert added.
"Parents have a big responsibility to ensure children are eating healthily because these are not good habits to be getting into at such a young age," said Dr Rees.
She added: "There needs to be official guidelines for parents or we will have an epidemic of health problems on our hands in a few years."
To reach the conclusion, the British Dietetic Association studied the lunches of 120 primary pupils across Cornwall. The typical packed lunch had 480 calories, while a cooked dinner of roast chicken, mashed potatoes, stuffing and vegetables or fish, chips and peas has 440 calories.
Packed lunches also average 834mg of salt and 7.2g of saturated fat, while traditional dinners contain 542mg of salt and 5.3g of saturated fat.