A British study found that 40% of cancer cases in the United Kingdom could be prevented if people changed their lifestyle.
Smoking rates in the United Kingdom are now understood to be falling at a rate of about 1% a year.
Prof Mel Greaves, a cancer biologist at the Institute of Cancer Research, in London, said the study was an "endorsement" of the idea that many cancers were potentially preventable.
"These figures show that we each can take positive steps to help reduce our individual risk of the disease", said Harpal Kumar, chief executive at the Cancer Research UK.
Cancer Research UK (CRUK) has said it toned down a major campaign warning of the dangers of obesity to avoid scaring overweight people, but has hit back at critics accusing the organisation of "fat-shaming". The research reported excessive weight to be the second biggest preventable cause of cancer.
Nearly two thirds of Britons are overweight or obese, which is up from half 25 years ago.
"The research clearly demonstrates the impact of smoking and obesity on cancer risk".
The grandmother of three, from Carlisle, said she weighed 20 stone at the time and admits she did no exercise.
It highlights the well-known dangers of smoking, with nearly half of all cases of cancer each year put down to tobacco.
It has now called for a ban on junk food adverts before the 9pm watershed.
Exposure to UV radiation from the sun and sunbeds caused around 13,600 cases of melanoma skin cancer in 2015 (3.8% of all cases), while drinking alcohol and eating too little fibre each caused around 3.3% of cancers. 'People regard being large as increasingly normal - that is a shift in cultural norms and acceptability.
However, overall the analysis found the proportion of preventable cases of cancer had fallen - from 42.7% in 2011 to 37.7%.
Rachel Rawson, of Breast Cancer Care, says: 'We can not turn a blind eye to the evidence that both being overweight and drinking alcohol increase the risk of developing breast cancer.
Researchers say it could be preventable by making a few simple changes in your daily routine.