The World Cancer Research Fund (WCRF) will hold its annual Fruity Friday campaign later this week.
The latest published Health Survey for England (HSE) for 2008 suggested that four in five children are not eating the recommended amount of fruit and vegetables.
And research by the Food Standards Agency released earlier this year showed that fewer than one in 10 teenagers were eating “five a day”.
The HSE found that, on average, boys aged five to 15 eat 3.1 portions of fruit and vegetables per day, and girls consume 3.3.
One in 14 boys and one in 25 girls did not eat any at all on the day of the research.
Nathalie Winn, from WCRF, said: “It is important that we encourage children to get into healthy habits as early in life as possible.
“This is because scientific research shows that eating a plant-based diet with plenty of fruits and vegetables, wholegrains and pulses probably reduces the risk of a number of types of cancer later in life.”
She said eating enough fruit and vegetables also helped people to stay at a healthy weight.
Obesity has been linked to a greater risk of certain cancers.