A survey of eight to 10-year-olds has found they are spending just 20 minutes a day being active when the recommended figure is an hour.
By the age of eight, girls are already becoming more sedentary than boys - a phenomenon previously noted in secondary school - and it could be due to a belief that sport is not "cool".
Female role models such as Olympic hopeful Jessica Ennis could help the problem.
In work published in the open access journal PLoS ONE, more than 500 eight to 10-year-olds wore activity monitors which gave analysts an accurate picture of how little time children spent exercising.
They were monitored for a range of actions from moving around, climbing stairs to running, playing games and skipping.
It revealed children spent only 4% of awake time in moderate to vigorous intensity physical activity, about 20 minutes per day, while the recommended amount for health is 60 minutes per day.
At the age of eight, girls were already less active than boys, which surprised researchers as the change happened earlier than previously thought.
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Older fathers tended to have less active children - possibly as a result of having more senior roles at work and less free time, or perhaps as they had a traditional, less hands-on view of parenting.
Children who took part in sports clubs outside of school were significantly more active than those who did not.
Newcastle University's Dr Mark Pearce, who led the study funded by the National Prevention Research Initiative, said: "Given the importance of physical activity in maintaining good health, we know we need to get our kids more active.
"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.
"Already at the age of eight, we are seeing girls being less active than boys.
"This is something which we know then gets worse as they approach their teenage years.
"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."
As to why the children of older fathers were found to be less active, Dr Pearce said: "We think there may be a variety of explanations for this such as older fathers reaching more senior posts and having to work longer hours or maybe seeing themselves in a more traditional role so spend less time in active play with their children."
Professor John Reilly from the University of Strathclyde, one of the researchers involved in this study said: "There is an urgent need for interventions, at home and at school, which will help primary school children become more physically active."
Dr Pearce said the Olympics could be a great advert for female sport.
"It would be a fantastic legacy for the Games," he said.
"Hopefully they can inspire children and encourage young girls to be more active."
One unexpected outcome of the research was a finding that children whose television-watching was restricted tended to be less active.
This may be because parents who do not allow children to watch much television do not want them to play sport either - if, for example, they were particularly keen on maximising study time.
It could be that those youngsters are not exposed to TV sport so are less interested, or it may be a statistical quirk, Dr Pearce said.
Inspiring ways to get active, by leading fitness coach, Tim Weeks...
How to 'think yourself fit' this spring by fitness trainer and coach Tim Weeks. Follow Tim on Twitter
If there is no passion in what you do, even the greatest motivation power wont get you over the first hurdle. Loving what you do makes the whole 'fitness experience' a lot of fun. Think of it like your job: hate your job and the week drags by. Love your job and nothing is ever a problem and motivation is constantly high.
Due to your natural peaks and troughs of ones mood (especially for women due to menstrual cycle), you need to have 4 or 5 different activities to choose from depending on your mood. If you really can't be bothered, having an easy option like a walk will mean you don't need superhero powers to motivate yourself to get off the sofa. Try as many different activities as you can until you find a few that 'click'.
However cliché this sounds, it works. This doesn't necessarily mean signing up to the London marathon or climbing Everest. It might be trying to complete a lap of the park jogging without walking.
Anyone can write a programme. The real talented trainers out there have something very special about them. Look at Jose Mourhino. Love him or hate him. He has an amazing ability to motivate people and unlock peoples potential to allow them to reach new heights. Having that 'motivator' at your side should be able to unlock that 'belief' within you that everyone has. Look for someone who is interested in your long term development. They can actually help you put all of these 10 items in this list into place. Not just getting ready for the beach in 4 weeks time.
'Stuff' happens in life that you have to deal with as you go along (good or bad). Build a support network of good people around you for when bad times arrive. It therefore means you don't always have to rely on will power. Having good people around you can actually kick start your motivation for you without you even realising.
If you get feedback from another person, their enthusiasm can help you feel valued. If you feel good about yourself, you naturally enter a self generating positive motivation cycle.
This may sound ridiculous but it can work. Put a chart on your fridge for 10 sessions. When you have completed all 10, treat yourself. Buy an item like a handbag, a pair of shoes or a book. The physical item is a nice reminder to yourself of how amazing you are and what you have achieved.
Sometimes having a friend giving you a kick up the bum is really helpful. You can actually motivate each other. Especially useful if one can't be bothered.
If you go over the top and set yourself totally unrealistic goals (eg I am going to exercise everyday) the low you feel is VERY low. You then need superhuman motivational powers to get back on track. Don't put yourself in the position in the first place.
If you exercise regularly, at some point you will hit a motivational lull. Therefore, when you go on holiday, enjoy yourself. Forget exercise. It will recharge your brain so you get back that hunger.