Office workers are advised to take regular breaks from their desks.
An hour’s “brisk exercise” each day offsets the risks of early death linked to a desk-bound working life, scientists suggest.
The analysis of data from more than a million people is part of a study of physical activity published in the Lancet to coincide with the Olympics.
Watching TV was found to be worse than sitting at a desk, probably because of associated habits like snacking.
Current NHS guidelines recommend 150 minutes of moderate exercise a week.
Being inactive is known to increase the risk of conditions such as heart disease, diabetes and some cancers.
It has been linked to 5.3 million deaths globally a year – compared with 5.1 million linked to smoking.
A cheat’s guide to staying active
The Lancet research says the global cost, for healthcare and lost productivity, is estimated at £57 billion per year.
To look at the the impact of activity and inactivity, researchers went back to the authors of 13 existing papers and asked all of them to reanalyse their data.
People were classed depending on how active they were – from the least active who did less than five minutes a day, up to 60-75 minutes a day for the most active.
Researchers then looked at how many people died during the follow-up period – between two and 14 years.
Those who sat for eight hours a day, but were physically active, had a much lower risk of premature death compared with people who sat for fewer hours a day, but were not active.
Sitting for a long time as well as being inactive carried the greatest risk.
Prof Ulf Ekelund, of the Norwegian School of Sports Sciences and the University of Cambridge, led the study.
He said: “For many people who commute to work and have office-based jobs, there is no way to escape sitting for prolonged periods of time. For these people in particular, we cannot stress enough the importance of getting exercise, whether it’s getting out for a walk at lunchtime, going for a run in the morning or cycling to work.
“An hour of physical activity per day is the ideal, but if this is unmanageable, then at least doing some exercise each day can help reduce the risk.”
But he admitted: “One hour’s moderate activity is substantially higher than current recommendations.”