People of all ages have been urged to exercise for at least 30 minutes a day five days a week.
A report from England’s chief medical officer says children should aim for 60 minutes of exercise each day.
Sir Liam Donaldson said physical activity is key to reducing the risks of cancer, heart disease and obesity.
He said people should make exercise part of their daily life – walking instead of taking the bus and taking the stairs instead of the lift.
‘Clear evidence’
Around three in four women and three in five men do not take enough exercise.
Sir Liam said there was clear scientific evidence to show exercise can improve health.
"The scientific evidence is compelling," he said.
"People who are physically active reduce their risk of developing major chronic diseases, such as coronary heart disease, stroke and type 2 diabetes, by up to 50% and the risk of premature death by about 20% to 30%."
Sir Liam said every adult should aim to do at least 30 minutes of moderate physical activity five days a week.
However, he said most people would need to exercise for between 45 and 60 minutes a day to prevent obesity.
Moderate physical activity should cause heart and breathing rates to increase. It may lead to sweating. However, it is not overly tiring – you should be able to continue for quite a while.
Sir Liam said the 60 minute target for children should include exercises that target the bones, muscles and overall flexibility at least twice a week.
The chief medical officer said people could hit their daily targets over the course of each day.
"The research demonstrates that the 30 minutes of physical activity necessary for health benefit can be built up in bouts of 10 minutes or more.
"For example, it can be made up of 10-minute brisk walks rather than catching the bus for short journeys."
The report is the latest government effort to try to encourage people to improve their own health.
A report for the government published earlier this year suggested the NHS could save billions of pounds a year if it did more to prevent ill-health rather than just treat it.
The government has since embarked on a national consultation exercise to find out what people think it should do to improve health.
Ministers will publish a public health White Paper later this year outlining what steps they intend to take.
Broad support
Health Secretary John Reid backed the report.
"This is a 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."
Paul Streets, chief executive of the Health Development Agency, said physical activity was "one of the cheapest, most effective ‘medicines’".
"Take 30 minutes activity a day to cut risk of heart disease by 50%, as well as reducing obesity, diabetes osteoporosis and colon cancer.
"In fact, being inactive is as serious a risk factor in heart disease as smoking," he said.
Jonathan Ellis of Help the Aged said: "Exercise is a crucial step that people of all ages must take to increase our chances of a long and healthy life."
