The Active Lives Children and Young People survey, published by Sport England, found that an estimated 27,100 children aged between five and 16 in Northamptonshire do less than 30 minutes of exercise a day on average.
A further 27,500 manage an average of at least half an hour a day, but less than an hour.
The Chief Medical Officer’s guidelines state that children should get at least an hour of physical activity every day of the week - but 82% of five to 16-year-olds in Northamptonshire fail to hit this target.
The Active Lives Children and Young People is the largest ever survey of its kind, based on responses from over 100,000 five to 16-year-olds during the academic year 2017-18.
Anything that raises your heart rate and makes you out of breath counts as physical activity - from organised team sports to playing outside.
It found that a third of children in England don’t manage an average of half an hour of activity a day, and less than 20% met the Chief Medical Officer’s target of an hour every day.
Sport England CEO Tim Hollingsworth said he was calling for system-wide change and “a national focus on the health and wellbeing of our nation’s children”.
He called the report “a big wake-up call for all of us”.
He said: “Parents, schools, the sport and leisure industry and Government all have a role to play in addressing and increasing childhood activity.
“These results tell us that what is currently being done to support them is not enough and change is required.
“We know that lots of factors affect behaviour and there is no single answer to the problem; but listening to young people and what they want is the best starting point.
“Our children deserve better, and Sport England is determined to play its part.”
Sports Minister Mims Davies said that the number of young people not doing enough exercise is “simply unacceptable”.
She said: “We know that an active child is a happier child, and efforts must be stepped up to encourage young people to live healthy, active lives.
“I know that Sport England are committed to making progress in this area, and our School Sport and Activity Action Plan will also ensure that all children have access to quality PE, sport sessions and clubs.”
Government guidelines state that children and young people should get half their daily hour of physical activity at school, and half out of school.
Clubs and sports teams that take place outside school hours count as out of school activities, even if they are school-run.
In Northamptonshire children were more likely to miss the target in school.
During school hours, 61% of students said they were active for less than 30 minutes a day, compared to 44% outside school.