Being overweight is now to blame for thousands more cases of bowel, kidney, liver and ovarian cancer than tobacco use.
And obese adults in the UK outnumbered smokers by two to one, a report by Cancer Research found.
This week, the head of the National Health Service (NHS) warned that Britain’s obesity epidemic was threatening to undo decades of medical progress on cancer survival rates.
Although smoking is still the UK’s biggest preventable cause of cancer overall, obesity now causes 1900 more cases of bowel cancer and 1400 more cases of kidney cancer a year.
Latest figures show that smoking causes 54300 cases of cancer a year, compared to 22800 caused by being overweight.
But experts believe that with plummeting smoking rates and soaring obesity, weight-related cases will soon overtake those linked to tobacco.
This week’s report reveals that obesity already causes a total of 3940 more cases of bowel, liver, kidney and ovarian cancers than smoking a year.
Excess body fat raises the risk of cancer because it sends out signals that can tell cells to divide more often and causes damage to build-up.
The Cancer Research UK report found there were about 13.4million non-smoking adults who were obese and 1.5million obese smokers, making a total of roughly 15million obese people in the UK.
This is more than double the number of smokers at just over 7 million.
Britain’s spiralling obesity crisis -driven by poor diets and sedentary lifestyles - means two in three adults are now overweight or obese.
Meanwhile, the number of smokers in the UK has plummeted by more than 2 million since 2011 following a major public health campaign.
Michelle Mitchell, Cancer Research UK’s chief executive, said: “Our children could be a smoke-free generation, but we’ve hit a devastating record high for childhood obesity, and now we need urgent government intervention to end the epidemic.
“They still have a chance to save lives.”
The charity is calling for new measures, including a 9pm watershed for junk food adverts on TV and online, and a restriction on promotional offers for unhealthy food and drinks.
Dr Lisa Wilde, of charity Bowel Cancer UK, said: “Scientists believe around half of all bowel cancers could be prevented by adopting a healthier lifestyle.
“Being overweight or obese, and carrying a lot of weight around your waist, can increase your risk.”