Healthy eating and exercise could reduce the risk of most common cancers by more than a third, says a study by the World Cancer Research Fund (WCRF).
The report also said that in developing countries like China and Brazil, such lifestyle practices, could cut cancer risk by almost a quarter.
These figures do not include smoking, which alone accounts for about a third of cancers.
The study is based on the 2007 release of 10 recommendations on how to avoid cancer, which include avoiding processed meats - including bacon and some sausages - eating less than 6 grams of salt per day, maintaining a healthy weight, and exercising every day.
"People think that somehow cancer comes from heaven, or Darwin, or from their parent's genes, but that's not always the case. A third are caused by smoking, and approximately a third are related to diet and physical activity," New Scientist quoted Michael Marmot, chair of the WCRF panel that produced the report, as saying.
For calculating the proportion of preventable cancers, the panel examined the biggest and most reliable studies to date on each of the 10 risk factors.
They then found a relative risk of developing cancer for each of the risk factor, which would be applicable to people not following the recommendations' advice.
After that they estimated the proportion of people in the US, UK, China and Brazil that indulge in this kind of "risky" behaviour.
After combining the values for the 12 most common cancers, they estimated that 39 percent of UK cancers are preventable, while 34 percent of US cancers, 30 percent of Brazilian cancers, and 27 percent of Chinese cancers could be avoided.
This means "that there are more actions that could be taken in the UK to reduce cancer than in the US, China or Brazil," said Martin Wiseman, medical and scientific adviser to the WCRF.
He further said that the consumption of alcohol is one of the biggest problems in the UK.
Although a glass or two of wine each day is good for the heart, "over that you get no additional benefit," said Marmot.
The study has called for the governments across the world to make it easier for people to lose weight and eat healthily.
It suggested that the above goal could be accomplished by increasing access to sports facilities and making it safer to cycle to work, and by reducing the cost of healthy foods so people are more inclined to buy them.
"Individuals decide for themselves what to eat, but let's make it easier for individuals and their families to make healthy decisions," said Marmot.