Analysis shows that being overweight causes around 1,900 more cases of bowel cancer than smoking in the UK each year.
Obesity now causes more cases of four common cancers than smoking, Cancer Research UK has warned.
Obese people now outnumber smokers by two to one.
The charity says that more needs to be done to help people lose weight to reduce their risk of cancer.
Although smoking is still the UK’s biggest preventable cause of cancer and carries a much higher risk of the disease than obesity, obesity is a cause of 13 different types of cancer, and trumps smoking as a leading cause for four of these types, the charity says.
Its analysis shows that being overweight causes around 1,900 more cases of bowel cancer than smoking in the UK each year.
It also causes 1,400 more cases of kidney cancer, 460 more cases of ovarian cancer and 180 cases of liver cancer.
National data shows that around one in three adults in the UK is obese, while around a third more are overweight.
It comes as Public Health England and the Office for National Statistics say future generations will be “smoke free” as the number of smokers has fallen to its lowest level since records began.
There are 5.9 million smokers in England, down by 1.8 million since comparable records began in 2011.
One in 10 children is obese by the age of five, rising to one in five by age 11.
The Cancer Research UK analysis used data from 2017 to show there were around 13.4 million non-smoking adults who were obese.
Meanwhile, 6.3 million adult smokers were not obese and 1.5 million adult smokers were.
The charity is calling on the government to act on its ambition to halve childhood obesity rates by 2030 and introduce a 9pm watershed for junk food adverts on TV and online.
Other measures should include restricting promotional offers on unhealthy food and drinks, it said.
Professor Linda Bauld, Cancer Research UK’s prevention expert, said: “There isn’t a silver bullet to reduce obesity, but the huge fall in smoking over the years – partly thanks to advertising and environmental bans – shows that government-led change works.
“It was needed to tackle sky-high smoking rates, and now the same is true for obesity.”
The charity’s analysis comes just days after it was heavily criticised for its new obesity campaign which is a billboard of a cigarette packet bearing the slogan “obesity is a cause of cancer too”.
It adds: “Like smoking, obesity puts millions of adults at greater risk of cancer.”
Many of the replies to a Cancer Research UK tweet unveiling the image accused the charity of “fat shaming”.