More than nine out of 10 under-fives are not active enough to be healthy, experts have said as they called on the Government to "embed" exercise in young people's lives.
Figures show just 9% of children aged two to four meet the current guidelines of having at least three hours of physical activity per day.
The British Heart Foundation National Centre for Physical Activity and Health (BHFNC), based at Loughborough University, is now calling for measures to support more exercise.
Experts said being active is proven to support brain development, improve bone health and muscular development, as well helping develop social and cognitive skills.
The centre wants to see exercise embedded in policies so it is part of everyday life at home, in early years settings such as nurseries, and in the community.
Health professionals should also track the physical activity levels of children during their early years, it said.
Elaine McNish, director of the BHFNC, said: "This manifesto outlines how we can create an environment for our children that encourages them and stimulates them to be active.
"This manifesto is a call to policy makers to ensure that early years settings are supported to create active environments.
"We know that active children are more likely to become active adults so it's vitally important to get it right at the beginning to give children opportunities to play from a young age and develop a lifelong love of being active."
Lisa Young, project manager for prevention and behaviour change at the British Heart Foundation, said: "We know that physical activity is an important component of a healthy lifestyle for everyone and the under-fives are no different.
"Developing a love of being active from a young age is important as we know active children become active adults, and active adults are healthier adults."