More than 10,000 cases of breast and bowel cancer could be prevented each year if people took more exercise, such as going for brisk walks, experts said today.
Just 45 minutes a day of activity at a moderate level could prevent about 5,500 cases of breast cancer in the UK.
At least 4,600 bowel cancer cases could also be stopped if people were moderately active for at least 30 minutes a day, five days a week, research showed.
Moderate activity is any type of exertion which gets the heart beating faster and makes people breathe more deeply.
Shorter bouts of exercise are also just as effective as longer sessions - it is the total time spent on activity that is important.
Today's calculations, from the World Cancer Research Fund (WCRF), show the importance of diet and exercise in lowering the risk of developing cancer.
Alongside brisk walking, other activities that would count include cycling at a leisurely pace, dancing, swimming at a leisurely pace, gardening and vacuuming combined with other housework.
According to the WCRF, exercise also cuts the risk of women developing womb cancer.
And because people who exercise tend to be more likely to keep a healthy weight, their risk of dozens of other cancers is lower than people who are overweight or obese.
Health experts warned last week that obesity was placing an "overwhelming" burden on the NHS as figures showed a 785% rise in weight-loss surgery.
Some doctors are "skirting around the rules" and not insisting on months of lifestyle change and pharmaceutical treatment before allowing patients to undergo surgery, specialists said.
Operations carried out for the most obese people in England have soared over the past five years, according to the NHS Information Centre.
Data for 2003/04 showed there were 480 procedures, rising to 4,246 in 2008/09.
Dr Rachel Thompson, deputy head of science at the WCRF, said people should aim to be physically active for at least half an hour every day.
"There is now very strong evidence that being physically active is important for cancer prevention," she said.
"Even relatively modest increases in activity levels could prevent thousands of cancer cases in the UK every year.
"These figures also show you do not have to go to the gym every day to benefit.
"You can reduce your cancer risk just by making small changes and this is highlighted by the fact that so many cancer cases could be prevented through something as simple as brisk walking.
"By taking up walking as a hobby or even walking to the shops instead of taking the bus or car, people can make a real difference to their health."
Henry Scowcroft, science information manager at Cancer Research UK, said: "You don't have to be an athlete to reduce your cancer risk.
"There's solid evidence that certain cancers - including breast and bowel cancer - are less common in people who do regular, moderate exercise such as brisk walking."Reuse content