By Michael Kahn

LONDON (Reuters) - Healthier living could prevent about a third of the most common cancers in rich countries and about a quarter in poorer ones, international researchers said on Thursday.

Better diets, more exercise and controlling weight could also prevent more than 40 percent of colon and breast cancer cases in some countries, according to the study which urged governments and individuals to do more to cut the number of global cancer deaths each year.

"At the time of publication, roughly 11 million people worldwide are diagnosed with cancer and nearly eight million people die from cancer each year," said Michael Marmot, who led the study from the World Cancer Research Fund and the American Institute for Cancer Research.

"However, cancer is mostly preventable."

The study involved 23 experts who analyzed both the incidence of 12 common cancers across the world and data on diet, exercise and weight to see how these factors contributed to kidney, mouth, lung, gallbladder and the other cancers.

The researchers found that healthier living would prevent 43 percent of colon cancer cases and 42 percent of breast cancer cases in Britain, and 45 percent of bowel cancer and 38 percent of breast cancer cases in the United States.

The findings follow the same groups' study in 2007 that showed how quickly people grow and what they eat are both significant causes of cancer.

They recommended -- in line with what health experts, including governments and the U.N. World Health Organization, have long been advising -- that people follow diets based on fruits, vegetables and whole grains and go easy on red meats, dairy products and fats.

The team also looked at China and Brazil as representatives of low- and middle-income countries, respectively.

Overall improving diet, exercise and weight would in the United States prevent more than a third of the 12 most common cancers -- which also included stomach, womb (uterus), prostate, pancreas and esophagus tumors.

This amounted to 39 percent of the cancers in Britain, 30 percent in Brazil and 27 percent in China.

"This report shows that by making relatively straightforward changes, we could significantly reduce the number of cancer cases around the world," Marmot said in a statement.

"On a global level every year, there are millions of cancer cases that could have been prevented and this is why we need to act now before the situation gets even worse."

(Reporting by Michael Kahn, Editing by Will Dunham)

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