Exercise is an important way of keeping fit and cutting body fat, which is linked to the risk of developing cancer
Data from the World Cancer Research Fund suggests that 20,000 cases of breast cancer and about 19,000 cases of bowel cancer could be stopped each year with small changes in lifestyle.
In 2013, there were more than 351,000 new cases of cancer in the UK.
The WCRF said 84,000 could have been prevented.
Head of research Dr Rachel Thompson said simple changes to diet and lifestyle could make ?a huge difference? in the battle against cancer.
?Even minor adjustments, like 10 to 15 extra minutes of physical activity each day, cutting down on alcohol, or limiting your intake of high calorie foods and sugary drinks, will help decrease your cancer risk,? she said.
She said that after cutting out smoking, being a healthy body weight was the most important thing people could do to cut their risk of getting cancer.
?There is strong evidence that being overweight or obese increases the risk of 10 cancers,? she said.
The link between a healthy lifestyle and the risk of developing cancer is well known, and this new data looks at preventable cases in 13 of the UK?s most common cancers.
For example, among men, 9% of cases of advanced prostate cancer could be prevented every year if men were not overweight or obese.
Lung cancer cases could be cut by 15,000 (33%) by getting people to stop smoking.
And 38% of breast cancer cases could be prevented, particularly in postmenopausal women, by increasing physical exercise and reducing body fat.
The WCRF also said that 2,200 cases of kidney cancer and 1,400 cases of pancreatic cancer could be prevented if people adopted a healthier lifestyle.
Prof Kevin Fenton, director of health and wellbeing at Public Health England, said the UK was currently behind on cancer survival rates compared with other European countries.
He said one major factor was that cancer prevention was not in the public consciousness.
?The link between tobacco and cancer is widely known and readily accepted by the public, but many are not yet fully convinced that healthy eating, regular exercise and not drinking alcohol, can lower your cancer risk.?