"Puppy fat" can have lifelong health implications and should not be dismissed as a temporary childhood problem, experts warn.
A poor diet is one factor which affects teens' weight
A Cancer Research study found more than a quarter of schoolchildren are overweight or obese by the age of 11.
The British Medical Journal study looked at over 5,000 children and found few who were overweight at 11 were a healthy weight by the age of 16.
Experts said it was essential children learnt healthy habits early.
The study of children, all at school in London, found 29% of 11-year-old girls could be classed as overweight or obese, compared to 20% of boys.
The figures were virtually the same when the children were followed up at the age of 16.
Few pupils gained unhealthy amounts of weight 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.
Advice to parents
The report also found that black girls were taller and heavier and had a larger waist circumference at the age of 11 than either white or Asian girls.
Among boys, those of Asian background were more likely to be overweight or obese than other groups but not by any significant amount.
Among pupils from more deprived backgrounds, 31% were overweight or obese at age 11.
Professor Jane Wardle, of Cancer Research UK's heath behaviour unit who led the study, said: "We have to abandon the idea that so called puppy fat doesn't matter and that it will just disappear when a child grows up.
"The evidence shows that children who are overweight or obese when they start secondary school at 11 are likely to leave education in the same condition.
"This means it is vital we work at preventing obesity in early childhood."
She added: "We know that overweight and obese children are most likely to continue carrying too much weight when they become adults and this will substantially increase their cancer risk as they grow older."
Professor Wardle advised parents whose teenage child was significantly overweight should handle the issue in a sensitive way, avoiding criticism and focussing on concerns over future weight gain rather than their existing weight.
She also said making healthy eating easier at home could help adolescents.
Professor John Toy, medical director of Cancer Research UK, said: "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."
He added: "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.
"A good diet includes plenty of fresh fruit and vegetables and high fibre foods like wholemeal bread.
"It is also important to encourage children to take regular exercise. Playing sports or running in the park are healthier occupations than playing computer games."