They risk being the couch potatoes of the future – the children who prefer playing computer games, watching TV or just lounging around to visiting their nearest skatepark or taking inspiration from Andy Murray and picking up a tennis racket.

They are not the majority, but they may be – and soon.

New research published on Thursday shows that almost half of all the country's seven-year-olds lead such sedentary lives that they do not even take the one hour of exercise a day which the UK's chief medical officers recommend as the bare minimum to boost their health and stop them becoming overweight or developing heart problems. While 50.8% of children of that age do have one hour of exercise, the other 49.2% do not meet the official recommendation.

The findings, published in the medical journal BMJ Open, have prompted renewed concerns about children's lifestyles and soaring childhood obesity, and whether the key pledge of last year's London Olympics – to "inspire a generation" to take part in sport – will ever be realised. It is already known from the government's National Child Measurement Programme in England that by the final year of primary school 33.9% of pupils are either overweight (14.7%) or obese (19.2%).

The four home nations' medical advisers believe all children and young people should do at least an hour's moderate to vigorous physical activity every day. But results from using accelerometers to measure the activity levels of 6,497 seven-year-olds over the course of a week show that in 2008-09, when the research was undertaken, exactly half (50%) of the pupils were sedentary for at least 6.4 hours a day.

Girls were particularly inactive: just 38% did the recommended hour of exercise, compared with 63% of boys. Children of Indian origin were the least active of seven ethnic groups, while just 43% of seven-year-olds in Northern Ireland managed the hour, compared with 52.5% in Scotland, the most active home nation.

Interestingly, children whose mothers had never worked or who were long-term unemployed were the most likely to do at least an hour's physical activity and were the least sedentary, while children from two-parent families were less active than those being brought up by just their mother.

The Royal College of Paediatrics and Child Health, which represents the UK's 11,000 specialist children's doctors, said it was worried about the trend towards so many children entertaining themselves indoors in front of devices rather than outdoors as previous generations did.

Prof Mitch Blair, the college's officer for health promotion, said: "It concerns me that half of all UK seven-year-olds are sedentary for six to seven hours every day and are failing to undertake the recommended daily minimum level of physical activity; at an age at which children should be moving around a lot more and enjoying active play instead of being glued to screens."

He called for limits on the number of fast-food premises allowed to open near schools, a complete ban on the advertising of junk food before the 9pm television watershed to reduce children's exposure to itand for all children to be taught how to cook nutritious meals at an early age, not just moves to boost exercise among young people. "We know obesity isn't going to go away overnight, but there are vital steps we need to be taking now to instil positive attitudes and behaviours so future generations lead healthier lifestyles, and as a result, lower the risk of developing serious obesity-related health conditions," Blair added.

Some schools have sought to encourage students to take part in activities that increase their heart rate by building climbing walls and offering more modern pursuits such as dance alongside traditional team games, which research shows turns some pupils off, especially girls. Thembi Nkala, a senior cardiac nurse with the British Heart Foundation, said: "This study shows us that far too many children are not nearly as active as they should be. If we want the next generation to grow up fit and healthy, we all need to do more to encourage children to be more active by providing a variety of fun and enjoyable activities that appeal to all groups."

The education secretary, Michael Gove, sparked huge controversy in 2010 by axing the £162m-a-year dedicated school sport grant, introduced by Labour, which schools had used to expand the range, quality and regularity of active pursuits they offered. As a result, much of the England-wide network of school sport partnerships disappeared and most of the school sports co-ordinators who had overseen a rise in participation lost their jobs. Sports stars joined furious headteachers, parents and pupils in protest, forcing Gove into a partial U-turn.

The Department of Health said the government was building on the Olympic and Paralympic legacy by investing £1bn in community sport. "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," said a health department spokesman.