Analysis of recent figures suggested Scotland has the largest number of preventable cancer diagnoses in the UK.
Research has revealed that nearly 2500 cases a week can be prevented if British people make simple changes in their lifestyle.
While smoking is still the biggest avoidable cause, accounting for three in every 20 cases, doctors say weight-related cancer is fast catching up.
This amounted to 7.5% of female cancer cases and 5.2% of male cancer cases.
Following a major new study of 2015 cancer data, the charity found that 37.7% of all cancers diagnosed in the United Kingdom each year could be prevented through lifestyle changes.
Across the United Kingdom, smoking remained by far the leading cause of preventable cancer, although it dropped from 19.4% in 2011 to 15.1%.
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Of the 360,000 new cases diagnosed in 2015, about 135,500 could have been prevented, according to Cancer Research UK.
"We took data from national surveys showing how common each risk factor is in the population, and data from the United Kingdom cancer registries showing how many cases of each cancer type there are".
In light of the findings, the charity has renewed its call to the Scottish government to introduce legislation to restrict bargain buy special offers on junk food.
He added that being severely overweight was "potentially the new smoking if we're not careful".
Processed meat was found to have caused 1.5% of cancers, while air pollution caused 1%.
Anna Gavin, director of the Northern Ireland Cancer Registry, which provided data for the research project, said: "We know so much about cancer, the big risk factors of smoking and diet, and the reductions that can come about by taking exercise, taking part in bowel and cervical screening when invited and having vaccinations for HPV".
"We have already reached a record low in the numbers of teenagers smoking, and halved the number of children being exposed to second hand smoke".