Some 40% of overweight or obese teenagers do not think that they are too heavy, a new study has found.

In recent years, a number of studies worldwide have suggested that people are increasingly unable to recognise overweight in themselves and other people. A report by Safefood in 2013 even found that when it came to overweight children, many Irish parents were in denial or were simply unaware that this was an issue for their child.

This latest study focused on teenagers. Researchers from the charity, Cancer Research UK, asked almost 5,000 young people aged between 13 and 15 years about their weight, and whether they thought they were too heavy, too light, or about right.

The teenagers' body mass index (BMI) was then measured to see if their perception of themselves was correct.

Some 20% of the young people were found to be overweight, while 7% were found to be obese. However among these, 40% thought that they were the correct weight.

The study also found that among those who were a healthy weight, most correctly identified this. However 10% incorrectly stated that they were too light, while 7% incorrectly stated that they were too heavy.

Girls were more likely than boys to think they were overweight when they were not.

"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," commented Prof Jane Wardle of Cancer Research UK.

The charity highlighted the fact that overweight and obesity increases the risk of up to 10 different types of cancer, including bowel and breast cancer.

"Overweight teenagers are more likely to become overweight adults at higher risk of cancer. 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. Making these changes as teenagers could help protect them from cancer as adults," commented Julie Sharp of the charity.

Details of these findings are published in the International Journal of Obesity.