Childhood obesity is determined before the age of five, researchers will tell ministers today.

Most of the excess weight gained by children before puberty has typically been put on before they are five, according to the study by the EarlyBird Diabetes Trust.

The researchers, based at the Peninsula Medical School in Plymouth, said this suggested that efforts to prevent childhood obesity should begin before school.

The study of 233 children from infancy to puberty also found that today's children are fatter than those of the 1980s.

The research assessed excess weight in a group of 223 children at birth, at age five and at age nine. Not only had both girls and boys gained much more fat by puberty compared with children 25 years ago, but most of the excess weight had been gained before five years of age. In girls, more than 90% of the excess weight had been gained before five and in boys more than 70%.

The study's director, Professor Terry Wilkin, said the findings could lead to a complete rethink of measures to tackle childhood obesity, in particular the emphasis placed on school sports.

"Interventions to prevent childhood obesity may be too late once children start school – as the majority of weight change occurs much earlier.  But the findings also suggest that the successful prevention of weight excess early in childhood is likely to be maintained, at least into puberty. Strategies to prevent diabetes might do better to focus on pre-school children, as the die appears to be cast by five years," said Wilkin.

The study's findings are to be presented to MPs today at a reception at the House of Commons.

The 12-year EarlyBird study is investigating the factors in modern-day childhood that are leading to record levels of obesity and the current epidemics of diabetes and heart disease in the developed world.