Experts say official guidelines on physical activity are being "ignored", creating a major public health challenge.
Official figures show many people are failing to meet the recommended level of two and a half hours of moderate activity each week.
"Physical activity is in all the Nice guidelines to show what we should be doing, and it's just being ignored."
Dr Mike Loosemore, UCLH.
They say just small improvements in overall activity levels would produce enormous benefits.
A lot of time is spent reporting on the damaging impact of problems such as obesity, smoking and excessive drinking. By comparison the threat posed by inactivity gets a lot less airtime - even though it causes millions of deaths from cancer, heart disease, diabetes around the world, and the situation is getting worse.
Global guidelines - adopted here in the UK - recommend two and a half hours of moderate activity a week for adults - things like cycling or brisk walking. They also call for a couple of sessions of "muscle-strengthening activity", which could include heavy gardening or yoga.
Official figures suggest about a quarter of men and half of women are missing the mark. But trials monitoring movement suggest the true picture is much worse - with only about one in fifteen men and fewer than one in thirty women - meeting the target.
"We have huge anti-smoking programmes. We have massive amounts of money being spent on obesity, and GPs are paid to check peoples diabetes and cholesterol status. We know that inactivity is a bigger health problem than all those things."
He says small improvements in activity could deliver huge health benefits.
"If we could get the population more active we could reduce the risk of bowel cancer by 60%. We could reduce the risk of diabetes by 50%. We could reduce the risk of breast cancer by 50%, reduce mild to moderate depression by the same as taking Prozac, we could improve bone health, reduce falls in the elderly, reduce Alzheimer's by 30%."
Dr Mike Loosemore is Head of Exercise Medicine at the Institute of Sport Exercise and Health, University College Hospital, London. He was chief medical officer for England at the 2014 Commonwealth game.