The report on some 1,000 schools from the Secondary Heads Association reveals a dramatic decrease in the hours devoted to physical education since the curriculum was introduced seven years ago.
Sport is suffering because schools are having to cram in 10 curriculum subjects. The UK is now bottom of the European league in the hours spent on PE. Downing Street and the Heritage Department are examining ways of revitalising school sport, including paying teachers for after-school activities.
Keith Smith, one of the report's authors and a retired head, said: "The Government should have a health warning with the national curriculum that this curriculum damages the health of your child."
Even in independent schools - which on average spend 50 per cent more time on PE for 14- to 16-year-olds than state schools - sport is being cut back as pressure to perform well in exam league tables mounts.
Ann Williamson, head of West Heath, a girls' private school in Sevenoaks, Kent, said PE time had been cut in her school because of the demands of the national curriculum.
In 1987, 72 per cent of 14- to 16-year-olds in state schools had at least two hours of PE a week but last year only 25 per cent achieved this. By 14 most state school pupils are receiving only an hour of PE a week. Weekend sport is also declining, with a quarter of state schools reporting a decrease since the heads' last survey four years ago.
The number of non-specialist PE teachers prepared to help with after- school sport continues to fall.
Schools said increased teacher workloads and the growing number of pupils with Saturday jobs were the main reasons for the fall in weekend and after- school sport.