The concept of childhood "puppy fat" that disappears in teenage years is a myth, researchers say. Children who are overweight or obese at 11 remain so through adolescence and probably into adulthood, a study in the British Medical Journal found.

Experts said obesity was established earlier than previously thought and dismissing the problem as puppy fat could have serious health implications later. More than a quarter of children in the UK are overweight or obese and the proportion has trebled in the past 20 years.

Researchers from the Cancer Research UK health behaviour unit at University College London followed almost 6,000 children from age 11 to 16. They found that at the age of 11 up to 19.3 per cent of the children were overweight and a further 6.9 per cent were obese. Over the next five years, 7 per cent of the children moved from being in the "normal" range to being obese and overweight. A further 7.6 per cent moved out of the overweight and obese categories into the normal range. Professor Jane Wardle, lead author of the study, said: "Children who are overweight or obese when they start secondary school are likely to leave education in the same condition."

While the proportion of children classed as overweight fell slightly there was a corresponding increase in those considered obese, so overall the number of pupils with a weight problem remained the same.

Doctors already see early signs of heart disease, diabetes and liver damage in children as young as 10 because of their weight.