A lack of exercise could be killing twice as many people as obesity in Europe, a 12-year study of more than 300,000 people suggests.
University of Cambridge researchers said about 676,000 deaths each year were down to inactivity, compared with 337,000 from carrying too much weight.
They concluded that getting everyone to do at least 20 minutes of brisk walking a day would have substantial benefits.
Experts said exercise was beneficial for people of any weight.
Obesity and inactivity often go hand in hand.
However, it is known that thin people have a higher risk of health problems if they are inactive. And obese people who exercise are in better health than those that do not.
The study, published in the American Journal of Clinical Nutrition, attempted to tease out the relative dangers of inactivity and obesity.
Obese versus Inactive
Researchers followed 334,161 Europeans for 12 years. They assessed exercise levels and waistlines and recorded every death.
"The greatest risk [of an early death] was in those classed inactive, and that was consistent in normal weight, overweight and obese people," one of the researchers, Prof Ulf Ekelund told BBC News.
He said eliminating inactivity in Europe would cut mortality rates by nearly 7.5%, or 676,000 deaths, but eliminating obesity would cut rates by just 3.6%.
Prof Ekelund added: "But I don't think it's a case of one or the other. We should also strive to reduce obesity, but I do think physical activity needs to be recognised as a very important public health strategy."
Prof Ekelund, who is based in Norway, is into cross country skiing and clocks up at least five hours of vigorous exercise each week.
However, he says all it would need to transform health, is brisk walking.
"I think people need to consider their 24-hour day.
"Twenty minutes of physical activity, equivalent to a brisk walk, should be possible for most people to include on their way to or from work, or on lunch breaks, or in the evening instead of watching TV."
The diseases caused by inactivity and obesity were largely the same, such as cardiovascular disease. However, type 2 diabetes was more common with obesity.
Commenting on the findings, Barbara Dinsdale, from the charity Heart Research UK, said: "This study once again reinforces the importance of being physically active, even when carrying excess weight.
"Changing your lifestyle is all good news for heart health, but physical activity is always easier to achieve and maintain without carrying the extra 'body baggage' of too much weight."
Prof John Ashton, president of the Faculty of Public Health, said changes were needed to make exercise easier.
"We need substantial investment in cycling infrastructure to make our streets safer.
"If more people cycled or walked to work or school, it would make a big difference in raising levels of physical activity."