Half of all UK seven year olds are sedentary for six to seven hours every day, and only half clock up the recommended daily minimum of moderate to vigorous physical activity, indicates research published in the online journal BMJ Open.
Girls, children of Indian ethnic origin, and those living in Northern Ireland are the least physically active of all seven year olds, the findings show.
The authors base their findings on a representative population sample of almost 7000 UK primary school children who were all part of the Millennium Cohort Study. This is tracking the long term health of around 19,000 children born in the UK between 2000 and 2002.
The duration and intensity of children's daily physical activity levels were captured for a full week between May 2008 and August 2009, using a gadget called an accelerometer, worn on an elasticated belt. The children only took this off when they bathed or went to bed.
UK guidelines on daily physical activity levels across the life course were revised in 2011. These recommend that children engage in moderate to vigorous physical activity for at least an hour every day, and that they spend less time sitting down, although no maximum has been specified for this.
The analysis showed that on average, across the entire sample, children managed 60 minutes of moderate to vigorous physical activity every day, and that they took an average of 10,299 steps.
But the accelerometer readings also showed that half the children were sedentary for six or more hours every day, and that half of them didn't reach the daily recommended exercise target.
Girls fared worse than boys, in terms of total physical activity, moderate to vigorous physical activity, and in the number of steps they took every day.
They were also more sedentary and less likely to meet their recommended daily exercise target than the boys. Just 38% of girls achieved this compared with almost two thirds of the boys (63%).
And children of Indian ethnic origin spent the least time in moderate to vigorous exercise, and took the fewest steps each day, while only one in three (33%) children of Bangladeshi origin met the recommended daily exercise minimum.
Among the four UK countries, children in Northern Ireland were the least active, with just 43% managing 60 minutes of moderate to vigorous physical activity every day, while children in Scotland were most likely (52.5%) to achieve the minimum daily target.
And while around 52% of children in England managed 60 minutes of moderate to vigorous exercise each day, there were regional differences, with children living in the North West (58%) the most likely, and those in the Midlands (46%), the least likely, to do so.
In an accompanying podcast, senior author Professor Carol Dezateux describes the gender differences in exercise levels as "striking" and calls for policies to promote more exercise among girls, including dancing, playground activities, and ball games.
The authors refer back to the legacy of the London 2012 Olympic Games, which promised to inspire a generation to take part in sport.
"The results of our study provide a useful baseline and strongly suggest that contemporary UK children are insufficiently active, implying that effort is needed to boost [physical activity] among young people to the level appropriate for good health," they write.
This is likely to require population wide interventions, they say, including policies to make it easier for kids to walk to school, in a bid to increase physical activity and curb the amount of time they are sedentary.
"Investing in this area is a vital component to deliver the Olympic legacy and improve the short and long term health of our children," they conclude.
Explore further: New generation electronic games boosts kids' physical activity at home
How active are our children? Findings from the Millennium Cohort Study, BMJ Open. DOI: 10.1136/bmjopen-2013-002893