As many as a third of teenage girls admitted in a recent poll that they were on a diet or had recently tried to lose weight.

The survey, conducted by supermarket chain Sainsbury's, worryingly found that nearly half of girls aged 13 to 18 said they aimed to consume less than 1,200 calories a day - 40 per cent less than is recommended for their age group.

Only 14 per cent of teenage boys admitted to dieting, but a quarter of those who did diet consumed less than 800 calories a day, only a third of their guideline daily amount.

The study showed a worrying lack of knowledge among teens about diet and healthy eating, with more than half not knowing how many calories they should be eating and despite 76 per cent saying they ate a healthy diet, only 15 per cent were eating recommended five portions of fruit and vegetables a day.

Sainsbury's nutritionist, Charlotte Parker said: "These results show that teenagers struggle to understand what constitutes a balanced diet.

"When this knowledge is combined with the issue of childhood obesity in the UK, it is clear that we need to help both teenagers and their families."