Three-quarters of parents whose children are overweight think that they are normal weight, new research has revealed.
A study by scientists at the University Medical Centre Groningen in the Netherlands found that obesity was similarly being mistaken.
Half of mothers and 39 per cent of fathers whose four and five-year-olds were classified as obese did not agree that their children were overweight.
Professor Pieter Sauer from the medical centre, publishing his findings in the journal Acta Paediatrica, said: "Overweight children are very likely to become overweight teenagers and adults, so intervening when they are aged between three and five could prevent weight problems later in life.
"It is vital that parents are aware of their children's weight if we are to prevent them becoming obese in later life. The fact that the parents in our study perceived their children to be lighter than their BMI indicated is cause for concern."
Obesity is medically defined as having a BMI over 30, while individuals are said to be overweight if they have a BMI above 25.
Independent advice on private healthcare