The UK has one of the world's least physically active populations, according to new research.
Just over 63% of people in the UK fail to meet recommended levels of physical activity, figures show.
The finding comes from a series of research papers that also warn that inactivity now causes as many deaths worldwide as smoking or obesity.
Britain's high inactivity rate contrasts with France, where fewer than 33% fail to do at least 150 minutes of moderate activity such as brisk walking, each week.
Even the United States has more active citizens - fewer than 41% take no exercise.
"Trends are leading to less not more activity," warned Dr Pedro Hallal from the Federal University of Pelotas, in Brazil.
The Lancet medical journal has published several studies underlining the risks of not taking exercise.
Another study, by doctors at Harvard Medical School, warns that 5.3 million people died in 2008 because they did less activity than recommended.
Overall, 9% of deaths were due to inactivity. But the UK's higher levels of inactivity resulted in double the death rate - 17%.
Dr I-Min Lee of Harvard Medical School in Boston blamed Britain's poor weather and sports infrastructure for low levels of activity.
But she told Sky News that people can - and must - change their lifestyles.
"You can't be an Olympian but you can do enough physical activity to be healthy.
"I do not mean for you to go out and run a marathon. If you can fit in 15-30 minutes of moderate activity a day - commuting, walking the dog, playing with grandkids, doing more vigorous housework...all these things will count."
Damian Korczyk has lost seven stone since joining a gym.
He said: "The best thing is to have a healthy life, don't smoke and exercise every day. I feel much better because I use the gym."