How children spend their after-school hours has a big impact on their levels of physical activity, new research has discovered, highlighting the need for children to be given more opportunities to play outdoors with friends.

In a study partly funded by World Cancer Research Fund, researchers at the University of Bristol have shown that hours spent outdoors with friends have the greatest positive affect on a child's level of physical activity. However, researchers found children spend most of their after-school time indoors, either alone or with parents.

For every hour spent outdoors with friends, the study found children doing an extra 17 minutes of physical activity. The government recommends children are physically active for at least one hour a day.

Dr Angela Page, who led the research, said: "We found that children spend most of their after-school time indoors and little time outdoors playing with other children, which makes the biggest contribution to the amount of physical activity they get.

"Building stronger neighbourhood community links between parents and children could restore the social norm of children playing outdoors and relieve some concerns parents may have about safety."

Dr Rachel Thompson, Head of Research Interpretation at World Cancer Research Fund, added: "There is evidence that physical activity protects against cancer, particularly bowel cancer, one of the most common forms of the disease in the UK. Getting into healthy habits during childhood is a big advantage when it comes to reducing the risk of getting cancer in later life as active children are more likely to become active adults.

"Being active is also beneficial in other areas of physical and mental well-being. This research demonstrates how children are most active when they're playing outside with friends, so we should be looking at ways we can encourage this in a safe environment."

To promote increased physical activity World Cancer Research Fund has launched the Move More Challenge to encourage families to take part in physical activities and games in an informal setting.

Amanda McLean, General Manager at World Cancer Research Fund, said: "In England, only about one in five children are reaching the recommended levels of physical activity, with adults faring better but still with room for significant improvement. This picture is broadly the same around the UK.

"A recent parliamentary commission emphasised the importance of being active at all ages and the Move More Challenge aims to help parents and children become healthier through a wide range of family-centred activities that encompass people's varied living circumstances."

The Move More Challenge is an email campaign whereby families that sign up are sent regular emails providing tailor-made suggestions for family activities. The suggestions range from a kickabout in the park to building a homemade obstacle course.

The challenge is launched on 12 May at the start of Cancer Prevention Week and the online tool is available from the start of half-time (17 May). As well as offering ideas for outdoor activities, it also provides indoor pursuits for when families don't have the time to go out.

The research - part of the Personal and Environmental Associations with Children's Health (PEACH) project and published in the International Journal of Behavioral Nutrition and Physical Activity - studied a sample of 427 children aged ten to 11 from the Bristol area. The intensity of their physical activity was measured with accelerometers while GPS determined whether they were indoors or outdoors. The children recorded who they spent their time with in diaries.

For both boys and girls being outdoors showed a stronger link with physical activity than being indoors. Time spent indoors with friends was also positively linked with physical activity but not as strongly as when outdoors (six minutes of physical activity per hour spent inside with friends).