Half of all seven-year-olds in the UK are not exercising enough to stay healthy – remaining inactive for between six and seven hours every day, a major study has revealed.

The researchers, who surveyed 7,000 children between May 2008 and August 2009, also found a "striking" gender gap, with girls far less active than boys. Only four in 10 girls achieved minimum recommended activity levels of an hour or more a day, compared with 63 per cent of boys.

The findings, in research published in the BMJ Open journal, come a day after official figures showed the Olympic legacy may have had a negligible impact on the activity levels of young children.

Despite hopes that the Olympic and Paralympic Games would "inspire a generation" to take up sport, a survey of 2,000 children by the Department for Culture, Media & Sport found that just under half of five- to 10-year-olds had not been encouraged to take up a sport because of the Olympics and, of those who did, only around a third said they had been encouraged "a lot".

Professor Carol Dezateux, of the University College London Institute of Child Health, a senior author of the BMJ study, said that the gender differences in exercise levels were "striking" and called for policies to promote more exercise among girls, including dancing, playground activities, and ball games.

"The results of our study… 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," the authors of the study write.

Population-wide interventions would be needed, they said, including policies to make it easier for children to walk to school, in a bid to increase physical activity.

"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 added.

Researchers measured children's activity rates by equipping them with accelerometers for an entire week, which were taken off only when the children went to bed or bathed.

Regional differences were also highlighted by the researchers. Well over half of seven-year-olds in Northern Ireland exercised for less than an hour a day, with 52 per cent of children in Scotland hitting the target.

Children living in the North-west of England were the most likely to exercise more, with 58 per cent doing an hour a day, while the most sedentary region was the Midlands, where only 46 per cent of seven-year-olds hit daily recommended exercise levels.

A third of children in the UK aged between two and 15 are overweight and between 14 and 20 per cent are believed to be obese. By 2030, research suggests, half the adult population of the UK could be obese.

Health experts warn that such high levels of obesity represent a "time bomb" for the NHS, with the inevitable rise in weight-associated conditions such as diabetes, heart disease and stroke set to rise dramatically. A government report published this week predicted that three in four adults could have a weight-related illness within two decades.

Diabetes and associated conditions already account for one pound in every 10 the NHS spends.

Professor Paul Gately, an expert in physical health and nutrition and the founder of the weight-management programme MoreLife, said that the school system was geared towards sports for children, but not towards encouraging children who might not be good at sport to stay active.

"We don't have a school system that promotes a greater level of physical activity. We encourage sporty kids, but not necessarily active kids," he said.

"There is a relationship between obesity and deprivation; it's not that clear-cut, but if you live in a densely populated area it makes you more likely to be obese, because it's not safe to play out, and you don't walk to school," he said. "Children with lower levels of learning also have higher levels of obesity; children with traumatic experiences are more likely to be obese. It… goes beyond the common picture of more children staying indoors and playing computer games."

John Steele, the chief executive of the Youth Sport Trust, said the findings were "concerning".

"If young people enjoy taking part in physical activity early on they will go on to lead active, healthy lifestyles, but if they have a bad experience, particularly at school, they could be put off for life," he said.