One in four overweight or obese teenagers don't see themselves as out of shape, and think their weight is "about right", research has revealed.
Conducted by Cancer Research UK, a survey of 5,000 13 to 15 year olds found 20% had a Body Mass Index (BMI) in the overweight category, and 7% were classed as obese.
However 40% of these teenagers thought they were a healthy size. Only 53% of boys and 68% of girls recognised the fact they were overweight or obese.
"Overweight teenagers are more likely to become overweight adults at higher risk of cancer," said Dr Julie Sharp, a researcher at the charity. "So it’s important that young people who are too heavy have support to be more active and make healthy changes to their diet - being aware that they are above a healthy weight could be a first step."
Being overweight increases the risk of up to 10 different types of cancer, including breast and bowel. It is estimated around 18,000 cancer cases in the UK are related to excess weight.
One in 10 teenagers of "normal weight" thought they weighed too little, with 7% saying they weighed too much.
Professor Jane Wardle, of the Cancer Research UK health behaviour research centre at UCL, said: "This study was a cause for celebration and concern. Young people who think they’re overweight when they’re not can sometimes develop devastating eating disorders, so we’re delighted that most of the normal-weight teenagers had a realistic view of their body size.
"But we need to find effective ways of helping too-heavy teenagers slim down and maintain a healthier weight, and it’s vitally important that we find out whether it helps if they are more aware of their weight status. There are no easy answers."