Cuts to the number of hours England's secondary school pupils spend doing physical education and sport "should be a matter of immediate national concern", the Youth Sport Trust says.
Figures released by the government show 4,000 hours were lost in state-funded schools in the last academic year.
There has also been a 12% drop since the 2012 London Olympics.
The children's charity says the latest data reveals a "further threat to the wellbeing of young people".
In March, the government announced that schools in England would be required to deliver a minimum of two hours of PE per week, and ensure equality of access to sport for girls and boys.
More than £600m of funding is to be delivered over two academic years.
Members of England's Euros-winning women's football squad - who on Sunday finished runners-up in the World Cup - campaigned for all girls to have equal access to school PE.
The team highlighted that only 63% of girls could play football in PE lessons, according to a Football Association (FA) campaign published last year.
