Nearly nine out of ten children fail to engage in the minimum amount of physical activity recommended to avoid long-term health problems, a study has found.

The study, for the Scout Association, found that about eight million children are spending less than the five hours a week of moderate physical activity that has been set by the Department of Health as a benchmark for maintaining a level of fitness to stave off diseases and problems from heart disease to insomnia.

The research suggests that although the vast majority of parents and children recognise the benefits of outdoor exercise, a number of factors still prevent children from swapping their computers for a game of football or other energetic leisure activity. They range from a lack of outdoor space and excessive school homework to peer pressure and parental fears about safety.

Peter Duncan, head of the Scout movement, said: "This report should come with a national health warning. Young people who spend more time outdoors are healthier that those who do not and at the moment the outlook is bleak."

The study follows a warning from a government minister that a generation of "battery-farmed children" was being raised because of an aversion to letting them stray far from home.