Half of all seven-year-olds do not get enough exercise - with girls far less active than boys according to new research.
Only 51% of all seven-year-olds in the UK achieve the recommended hour of exercise every day.
That figure is broken down to just 38% of girls hitting the target, compared with 63% in boys.
Half of this age group is also sedentary for an average of 6.4 hours or more every day, experts have found.
The research, published in the online journal BMJ Open, found that children of Indian origin and those living in Northern Ireland are among the least physically active of all seven-year-olds.
Experts from the University College London's Institute of Child Health examined data for 6,497 children.
The youngsters wore an accelerometer to measure exercise levels which was attached to an elastic belt round their waist. It was only removed when bathing or when the children went to bed.
In total, the experts were able to record 36,309 days of data based on the children wearing the accelerometer for at least 10 hours a day over the course of a week.
Girls were more sedentary and less active than boys 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 the recommended 60 minutes, while children in Scotland were most likely (52.5%) to achieve the target.
Around 52% of all children in England managed the 60 minutes but there were regional differences.
The researchers wrote: "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."
Senior author Professor Carol Dezateux, from the Institute of Child Health, called for policies to promote more exercise among girls.
A Department of Health spokesman said: "We have committed to giving primary schools £300m of ring-fenced funding to improve PE and sport, and help all pupils to develop healthy, active lifestyles, and have invested a further £3m to extend Change4Life School Sports Clubs to areas with the highest childhood obesity."