The findings raise fresh concern over our "dangerously sedentary" lifestyles, which put people at risk of type 2 diabetes, cancer and early death.
It also discovered the greatest barriers to exercise were busy working lives (cited by one in five people), followed by family commitments (cited by 18 per cent of women and 12 per cent of men).
Social media can motivate younger people to be more active, although it has less influence on older age groups.
More than 40 per cent of 18-24-year-olds said friends and celebrities posting pictures on Instagram of them exercising had a positive effect on them, but the figure for all age groups dropped to 28 per cent.