Researchers at the Universities of Strathclyde and Newcastle found that some eight to 10-year-olds were active for only 20 minutes a day.
Their study suggests girls are less active than boys. The findings are published in the journal PLoS One.
The scientists gave 508 primary school children advanced pedometers to measure their physical activity levels over a week.
They found that just 4% of waking time, or 20 minutes, was spent doing moderate to heavy vigorous activity.
They said 60 minutes a day was recommended.
Dr Mark Pearce, from Newcastle University, told the BBC he was surprised by the low activity levels "and even more surprised that girls were even lower".
He added: "Activity drops in teenage years and if its this low at eight, there's not much further to fall."
Role models on TV?
Older fathers seemed to have less active children and, curiously, parents who restricted television access also had less active children.
The researchers said it was an unexpected result which may have been a quirk in this group of school children. One idea is that television acts as a role model, so watching the European Championships may inspire children to try to emulate the likes of Wayne Rooney in the playground.
Prof John Reilly, from the University of Strathclyde, said: "There is an urgent need for interventions, at home and at school, which will help primary school children become more physically active."
The researchers said it was important that parents did more to get their children into sport, but that it was also the responsibility of schools and education authorities.
Dr Pearce said: "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."
Article taken from the BBC news website: LINK