Children must be targeted before their eighth birthdays in order to counteract an obesity "time bomb", according to researchers at the universities of Strathclyde and Newcastle.
Their study reveals that children are far less active than they should be, and that girls are already becoming more sedentary than boys by the age of eight.
Some 508 children aged 8-10 wore "activity monitors" and were assessed for a range of actions, from moving around and climbing stairs to running, playing games and skipping.
The researchers found that the children spent only 20 minutes a day in moderate to vigorous-intensity physical activity, a third of the recommended 60 minutes.
Girls were already less active than boys at age eight - a trend that has often been assumed to emerge in the secondary school years.
Other findings include: older fathers tended to have less active children; children who took part in sports clubs outside school were "significantly" more active than those who did not; and parents who restricted access to television had less active children - although the study acknowledges other research associating increased television viewing with an increase in body fat.
"There is an urgent need for interventions, at home and at school, which will help primary school children become more physically active," said Professor John Reilly, of the humanities and social sciences faculty at the University of Strathclyde.
"Given the importance of physical activity in maintaining good health, we know we need to get our kids more active," said Newcastle University's Mark Pearce, who led the study. "What we hadn't known until now is how young we need to be catching them, or the reasons that lay behind their lack of activity."
He added: "One of the important things is that most girls don't see sport as cool. We need to be tackling these issues earlier by encouraging girls to exercise, by providing a wider range of opportunities than are currently on offer and by ensuring they see positive female role models, particularly in the media."
The research formed part of the Gateshead Millennium Study. It was funded by the National Prevention Research Initiative, and has been published in the open access journal PLoS ONE.
Tennis coach Judy Murray has blamed a lack of PE in schools for creating a generation of "overweight and uncoordinated" youngsters.
The captain of the Great Britain Federation Cup team and mother of world number four player Andy Murray was reported to have been shocked by the number of obese pupils she encountered during a recent tour of secondary schools to promote her Set4Sport programme, which aims to improve coordination and grassroots participation in sport.