School days should be extended by two hours for extra-curricular activities including sport, a think tank has suggested.

Physical activities and sports should be used as a tool to improve physical and mental health, the Centre for Social Justice (CSJ) said.

Its report – A Level Playing Field – calculates the cost of rolling out an extended school day for children in Years 7 to 11 at £1.5 billion, which could be funded by taxpayers, sponsorships and “small contributions from parents”.

This is much less than the CSJ’s estimation of the cost of inactivity among young people, which stands at £53.3 billion during their lifetimes, and it suggests schools should put on a “carousel of sporting activities”, including team-based and competitive sports, and individual exercise such as dance.

The CSJ said independent schools currently have a “large emphasis … placed on extra-curricular competitive sport” and that these schools which have charitable status should be obliged to share their facilities.

It said: “This would help independent schools to demonstrate their fulfilment of the public benefit test required to retain their charitable status, as well as narrowing existing sports participation inequalities that typically fall along deprivation lines.”

Currently, most after-school provision is teacher-led, but the CSJ said the extended school day proposals could involve schools working with community sports clubs, charities, or in partnership with other educational establishments.

An increase of 10% in those children and young people attaining the recommended levels of physical activity could reduce the cost of physical inactivity by around £8 billion over the course of the lives of today's 11 to 25 year-olds

The report said: “Physical inactivity among today’s young people is estimated to cost £53.3 billion during their lifetimes, through a costly burden of diseases related to inactivity and lower quality of life, and life expectancy.

“An increase of 10% in those children and young people attaining the recommended levels of physical activity could reduce the cost of physical inactivity by around £8 billion over the course of the lives of today’s 11 to 25 year-olds.”

Other ideas for England in the report include rolling out the Daily Mile – a scheme from Scotland which encourages schools and nurseries to take children outside to jog or run for 15 minutes – as well as following a move in Scotland and Wales which permits school facilities to be used by pupils after hours.

The law should also be amended to allow the Education Secretary to set a minimum number of hours per pupil per week of physical activity, the CSJ said.