Childhood obesity is set before age five, government ministers will hear today.
Compared to children in the 1980s, today's youngsters are fatter and most of their excess weight gain happens before school age, researchers will say.
This suggests initiatives to prevent childhood obesity should be started before school, say the authors.
The EarlyBird Diabetes study of 233 children from birth to puberty is being published in the journal Pediatrics.
One in four children aged four to five in England is overweight, latest figures show.
At birth, the children in the study were of similar weight to babies 25 years ago, but had gained more fat by puberty compared with children of the same age in the 1980s.
Weight at five years bore little relation to birth weight, but closely predicted weight at nine years old.
Lead researcher Professor Terry Wilkin, of the Peninsula Medical School, Plymouth, said: "When they reach the age of five the die seems to be cast, at least until the age of puberty. What is causing it is very difficult to know."
He said there must be a factor now that was not there 25 years ago which is making today's children obese. And, given the young age, this is likely to be in a child's home rather than school environment and linked to upbringing rather than schooling. Rather than lack of physical exercise, he believes diet could be to blame.
"It is entirely possible that the calorie density of food and portion sizes could be higher."
Professor Wilkin said there had been a lot of focus on school meals, PE time, school runs, television viewing and computer games in the development of childhood obesity, but these are all issues for school age children.
He said strategies to prevent childhood obesity and its associated health problems, such as diabetes, might do better to focus on pre-school children.
David Haslam, of the National Obesity Forum, said: "It is never too late or too early to intervene. The earlier the better in terms of long-term outlook."
He said early childhood obesity was likely to be down to environment and learned behaviours.
