The idea that "puppy fat" in children disappears as they progress to adolescence is a myth which may put the future health of children at risk, a study suggests.

Cancer Research UK researchers looked at nearly 6,000 school children as they developed into young adults.

They found that children who had weight problems at 11 were still overweight by the age of 16.

The study revealed that 29 per cent of 11-year-old girls were overweight, compared to 20 per cent of boys.

The figures were nearly the same when the children were followed up five years later.

The report indicates that few pupils moved into the overweight or obese category between the ages of 11 and 16.

But equally, few of the children who were already obese or overweight dropped to a healthy weight during those years.

The report also found that black girls were taller and heavier, had a higher Body Mass Index and a larger waist circumference at the age of 11 than either white or Asian girls.

Prof Jane Wardle, Cancer Research UK: "Children who are obese when they enter secondary school will very likely leave it obese."

For boys, however, ethnicity made little difference to excess weight levels.

The findings were less clear cut for economic status.

Thirty five per cent of the most deprived girls were overweight or obese at age 11, compared with 28 per cent of other girls.

Professor Jane Wardle, of Cancer Research UK's heath behaviour unit who led the study, said: "Children who are obese when they enter secondary school will very likely leave it obese.

"More monitoring is crucial if rising tides of obesity are to be tackled effectively."

Professor John Toy, medical director of Cancer Research UK, added: "We know that being overweight or obese increases the risk of cancer in adulthood.

"This study shows that a worryingly high number of children as young as 11 have already established a pattern of weight gain that can lead to health problems including cancer in later life.

"It is essential that we help parents to learn about the importance of cultivating healthy eating habits in children to give them the best possible start in life."

Previous research has shown that adolescence is a key time, since excess weight during teenage years pre-disposes adults to continued weight problems.

The current study is published in the British Medical Journal.