More than nine in 10 older children are not eating the recommended five portions of fruit or vegetables every day, a charity has warned.
Only 8% of 11- to 18-year-olds are achieving the target, according to the World Cancer Research Fund.
Overall, youngsters in this age group consume 2.8 portions each.
Figures collated from the National Diet and Nutrition Survey, which assesses the diet of the general population of the UK, showed that 8% of boys in this age group managed their five-a-day compared with 9% of girls.
The charity, which has launched a new recipe website, said the figures were "seriously worrying" as a diet low in fruit and vegetables could increase a person's risk of lung, mouth, pharynx and larynx cancer in later life.
Meanwhile, only 29% of adults aged 19 to 64 manage to eat five portions a day, compared with 36% of over-65s.
A recent study, published in the International Journal of Epidemiology, found that consuming 10 portions of fruit and vegetables every day could significantly reduce the risk of heart disease and cancer.
While eating the recommended five a day still helped reduce disease risk, the highest benefits were seen when people consumed 10 portions, researchers found.
World Cancer Research Fund says its new website, Real Recipes, provides meal tips which have been checked by qualified nutritionists and are backed by the charity's cancer prevention evidence.
Sarah Toule, head of health information at the charity, said: "These figures are seriously worrying, so we decided to do our bit to encourage people of all ages to fill up on healthy, homemade dishes featuring lots of fruit and veg.
"We know from our global research that eating a healthy diet and maintaining a healthy weight are two major steps you can take to reduce your risk of cancer.
"The younger these healthy habits are developed, the easier they are to maintain into adulthood.
"Real Recipes is a site where you can trust all the recipes to be healthy, as well as helping reduce your risk of cancer."
Dr Louis Levy, head of nutrition science at Public Health England, said: "Children need a variety of at least five portions of fruit and vegetables a day.
"It is vital for a healthy diet and to help children avoid becoming overweight, a problem which they are likely to carry into adulthood.
"Managing portion sizes and reducing their intake of salt, saturated fat and sugar will help children maintain a healthy weight and minimise the risk of heart disease, type 2 diabetes and some cancers later in life."