Parents should not give their children ham sandwiches in their packed lunches, a charity warned today.
Youngsters need to get into the habit of avoiding processed meats like ham, salami, hot dogs and bacon, it said.
According to the World Cancer Research Fund (WCRF), there is "convincing evidence" that eating processed meat increases the risk of bowel cancer.
While there is no specific research on eating the meats in childhood, the charity said the evidence in adults was too strong to ignore.
Children should therefore adopt a healthy eating pattern from the age of five and avoid processed meat, it said.
Marni Craze, children's education manager for the WCRF, added: "If children have processed meat in their lunch every day then over the course of a school year they will be eating quite a lot of it.
"It is better if children learn to view processed meat as an occasional treat if it is eaten at all."
The WCRF is also urging parents to avoid lunchbox fillers that are high in fat and calories.
Foods such as sugary drinks can cause weight gain, and obesity in adulthood is linked to cancer, it said.
The charity warned that parents may be misled by packaging or where a product appears on a supermarket shelf or website.
For example, Sainsbury's lists Peperami salami snacks in the "kid's lunchbox" section of its website, even though it is 44% fat, it said.
Just one 25g Peperami stick also contains 126 calories and 11g of fat.
Ms Craze said: "With the large number of overweight and obese children in the UK, it is important that parents check the nutritional information on food to see if it is high in calories.
"If children are regularly eating high-calorie foods or sugary drinks then they are more likely to become overweight.
"Putting ham or high calorie snacks in your child's sandwich might seem like a convenient option, particularly for parents who do not have a lot of time to prepare their child's lunchbox.
"But packed lunches are a part of a child's diet that is relatively easy to control and it does not have to take too much time or effort to prepare a healthy lunch.
"For example, putting some salad into a sandwich will count towards the five portions of fruits and vegetables children should be eating every day.
"And a small fruit juice instead of a fizzy drink will also give a portion.
"Also, chicken that has not been processed, fish, houmous or low fat cheese are easy and quick alternatives."
Scientists estimate about 3,700 bowel cancer cases could be prevented each year in the UK if everyone ate less than 70g of processed meat a week, which is roughly the equivalent of three rashers of bacon.
As well as avoiding processed meat, the WCRF also recommends limiting intake of red meat to 500g (cooked weight) per week.
A recent survey showed that two thirds of people in Britain were unaware that eating processed meat increases the risk of cancer.
The WCRF runs the Great Grub Club website ( www.greatgrubclub.com), which encourages children to adopt healthy habits and includes a section for parents.