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.