Almost 37,000 deaths in England could be prevented every year if people walked for just two-and-a-half hours every week, according to a new report.
A further 12,000 cases of people needing emergency hospital treatment for heart disease could be stopped and almost 7000 cases of breast cancer could be prevented.
The study, from the Ramblers and Macmillan Cancer Support, also showed that just 150 minutes a week of moderate activity - such as cycling or fast walking - could prevent almost 5000 cases of bowel cancer and almost 295,000 cases of diabetes.
'People who stay active are less stressed, sleep better, have a 30 per cent lower risk of getting depressed and reduce their risk of developing dementia,' the report - Walking Works - said.
It added: 'Physical inactivity now rivals smoking as one of the nation's biggest health problems. Today it's responsible for 17 per cent of early deaths in the UK.
'Physical inactivity is the principle cause of a huge number of common health conditions including 10 per cent of heart disease cases, 13 per cent of type 2 diabetes cases, 18 per cent of colon (bowel) cancer cases and 17 per cent of breast cancer cases.
'But that's not all. Research reveals that being inactive increases your chances of developing cancer, heart disease or having a stroke by 25-30 per cent and can take three to five years off your life.'
Britain's National Health Service recommendations are for adults aged 19 to 64 to try and be active every day. This includes at least 150 minutes of moderate-intensity aerobic activity such as cycling or fast walking every week.
The report comes after a study published last week found that postmenopausal women who walk for an hour a day can cut their chance of breast cancer by 14 per cent. Those who were more active cut the risk by 25 per cent.