Sedentary lifestyles are linked to increased risks of heart disease, diabetes and cancer, researchers found, but activity - such as brisk walking - could counter the higher likelihood of early death linked with sitting for eight or more hours a day.
Such inactivity is estimated to cause more than 5 million deaths a year - almost as many as smoking, which the World Health Organization (WHO) says kills 6 million a year.
Giving details of their findings at a briefing in London, the international team of researchers warned there has been too little progress in tackling a "pandemic of physical inactivity".
Ulf Ekelund, a professor at the Norwegian School of Sports Sciences and Cambridge University, said that WHO recommendations for at least 150 minutes of moderate exercise a week was probably not enough. A quarter of adults worldwide do not meet even the WHO's recommendations.
"You don't need to do sport or go to the gym ... but you do need to do at least one hour a day," he said, giving walking at 5.6 km an hour (km/h) or cycling at 16 km/h as examples of what was needed.
People who sat for eight hours a day but were otherwise active had a lower risk of premature death than people who spent fewer hours sitting but were also less active, suggesting that exercise is particularly important, no matter how many hours a day are spent sitting.
The greatest risk of premature death was for people who sat for long periods of time and did not exercise, according to the findings, published in The Lancet on Wednesday.
In another of the series of four studies, researchers estimated healthcare costs and productivity losses for five major diseases linked to lack of exercise - heart disease, stroke, diabetes, breast cancer and colon cancer - cost $67.5 billion globally in 2013.
Melody Ding of the University of Sydney, who led this part of the research, said the costs occur largely in wealthier countries, but as poorer countries develop, so too will the economic burden of chronic diseases linked to inactivity.
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