Four in ten adults aged between 40 and 60 are not achieving the recommended ten minutes of continuous brisk walking in a typical month.
This is the finding of Public Health England (PHE), which said more than 6.3 million adults in this age range are not walking briskly for at least ten minutes as often as they should.
The organisation is therefore concerned that many Britons could be missing out on important health benefits.
As a result, adults have been urged to build ten minutes of continuous brisk walking into their day in order to improve their health.
Indeed, PHE believes at least one brisk ten-minute walk can reduce the risk of early death by 15 per cent and stave off conditions such as type 2 diabetes, cardiovascular disease, dementia and some cancers.
Furthermore, it warned that the current "epidemic" of physical inactivity among adults is contributing to one in six deaths in the UK and costing the NHS almost £1 billion a year.
Dr Jenny Harries, deputy medical director at PHE, commented: "I know firsthand that juggling the priorities of everyday life often means exercise takes a back seat.
"Walking to the shops instead of driving or going for a brisk ten-minute walk on your lunch break each day can add many healthy years to your life."
Professor Sir Muir Gray, a clinical adviser for the PHE's One You campaign, added that the lifestyle changes being recommended are "easily achievable".
Figures from PHE indicate that British adults are 20 per cent less active than they were 50 years ago and, on average, walk 15 miles less a year than they did 20 years ago.