Sir Liam has released a report on physical activity and health, At Least Five A Week.
Smoking and unhealthy diet have long been established as major causal factors for chronic disease but the report says that inactive living is equally important.
Sir Liam Donaldson said:
"People need to stay active over the whole of their lives if they are to stave off the threat of obesity and killer diseases like cardiovascular disease, type II diabetes and cancer.
"We are moving less than our parents and grandparents. This is a major risk factor for the nations health. We need to combat the couch potato culture, and this means building moderate everyday physical activity into our lives.
"It is not about spending hours and hours in the gym, but it is about finding ways to build activities into our daily lives.
For general health benefit, adults should achieve a total of at least 30 minutes a day of at least moderate intensity physical activity, on five or more days of the week.
This report will be considered by Ministers when they draw up the forthcoming White Paper on Public Health, following the public consultation exercise, currently under way.
John Reid, Health Secretary, said:
"This is an excellent report which clearly sets out the importance of a more active lifestyle. The challenge for all of us; Government, business, the voluntary sector and individuals themselves, is how we achieve that."
Sir Liam said the recommended levels of activity could be achieved either by doing all the daily activity in one session or through shorter bouts of activity of 10 minutes or more.
"For example, an adult may take a daily brisk walk or cycle to work and children could be encouraged to walk to school, in addition to two or three weekly leisure activities such as swimming, football, or gym. All activity can help prevent obesity, so people should make the most of all small opportunities to be active such as using stairs and doing the gardening."
The report highlights that up to two-thirds of men and three quarters of women don't take enough physical activity for a health benefit; a quarter of adults and six per cent of 2-20 year olds are obese; and the cost of inactivity direct costs of treatment and indirect costs caused through sickness absence is an estimated £8.2bn annually.