We're a nation of fatsos - everyone knows that. Kids don't exercise enough and are getting much too hefty. But did anyone realise it was quite as bad as it is? A new study in the Archives of Disease in Childhood reveals the shocking news (even in the current coach- potato climate) that only one in 40 11-year-olds is doing the minimum 60 minutes' exercise a day.

According to the research - which involved 5,595 children in the Avon area - 95% of boys and 99.6% of girls fell short of the recommended levels of exercise. And the research is particularly significant because it fitted the kids with accelerometers - devices to measure precisely how much and how often and with what sort of intensity they moved about over seven days. Previous research has suggested a much higher proportion of youngsters are getting enough exercise. "But most of that previous research was based on questionnaires, which aren't as accurate as the accelerometer, which monitors actual activity," says Dr Calum Mattocks, a research associate on the Avon Longitudinal Study of Parents and Children, known as the Children of the 90s study, which involves 14,000 children altogether.

Given that the new figures are likely to be more accurate than previous research, what is especially worrying is the plummeting exercise levels of girls. Although it is widely acknowledged that they tend to be less inclined than boys to play sport as they get older, it now seems that many girls are inactive as early as the start of secondary school. "Girls are doing less exercise than ever," says Mattocks. "It's especially worrying that girls' activity levels peak at this age - far short of what they should be - and then decline further through adolescence."

What is even more worrying is that many experts believe the target of an hour's daily activity is almost certainly too low. That is the view of Dr Richard Winsley of the children's health and exercise research centre at Exeter University. And there is no reason, he points out, why they shouldn't be doing more. "When you think about playtime, getting to and from school, and running around at home, it should be quite possible to get an hour's exercise. Given the opportunity, children relish the opportunity to run around."

Others agree. Professor Ken Fox of the department of health and exercise science at Bristol University says that, ideally, children should be involved in active play (that means anything that does not involve sitting down) for at least another 60 minutes a day. School sport, says Fox, plays its part. About 80% of secondary school children now do two hours of physical activity a week - 30% more than in 2003-04. By 2008, the government wants to see 85% of secondary school pupils doing that much, and by 2020 all those at secondary school should be offered four hours of sport every week.

Meanwhile, schoolchildren are getting fatter and - according to the Health Survey for England - by 2010, around one fifth of boys and girls are expected to obese. So what else can be done? Like most experts, Fox stresses that it is daily habits outside school hours that really make a difference. What has changed markedly in the two to three decades since waistlines began expanding at a faster rate than anywhere else in Europe is the home environment, the nitty-gritty of everyday living.

"The root of the problem is that children spend far longer sitting down than they used to do," says Louise Sutton, principal lecturer in health, nutrition and exercise at Leeds Metropolitan University. "They are not actually eating any more calories than children were 10, 20 or 30 years ago, but they are expending fewer."

Being active during childhood is not just an issue of weight gain and related image problems. "Diseases like diabetes and osteoporosis are striking at a younger age and that has a lot to do with a lack of exercise," says Sutton. "But the side effects of laziness are not just physical. They have a wide-reaching impact on development." Surveys by the Qualifications and Curriculum Agency (QCA) have found that children who regularly play sport or exercise achieve higher grades in school and have better behaviour records. Another study by researchers at Oxford University found that exercise, particularly in groups, improved social skills, self-esteem and mood in young people.

So what can you do to get kids off the sofa? Try these ...

· Buy them sporty toys: skipping ropes, skateboards, basketball nets, pogo sticks, inline skates, baseball or cricket sets. Inject a sense of excitement with something fun and different.

· Let them play: in the UK, no specific exercise guidelines are set for children under the age of six, but they are likely to be in line with those set by the American Academy of Paediatrics (AAP), which recommends only 15 minutes of "structured activity", ie specific games such as football, throwing a ball or swimming - for the under-fives. Aside from that, encourage plenty of "free play" - digging in a sandpit or the garden, dancing to music or building bricks all count towards activity at this age.

· Set an example: countless studies have shown that the more active the parents, the more active their children. Foster an active family lifestyle. Swim or cycle together, play football, or try ice-skating or rollerblading.

· Stage mini-sports events: get your children and their friends to take part in fun mini-triathlons in the garden or park. Any activities can be included: set out a route for them to run or cycle; time how fast they can sprint between lampposts; time them dribbling a football between cones or see how many baskets/goals they can score in two minutes.

· Walk them to school: if an hour a week were switched from car travel to walking, it would prevent the average person from gaining 12.7kg over a decade.

· October is international walk to school month; for more information see walktoschool.org.uk