Being overweight is cause of 13 different types of the disease, and trumps cigarettes as leading cause for four of these types.
Obese people now outnumber smokers by two to one, Cancer Research UK has warned, as it said obesity causes more cases of some cancers than cigarettes.
The charity said more needs to be done to help people lose weight to reduce their risk of cancer.
Smoking is still the UK’s biggest preventable cause of cancer and carries a much higher risk of the disease than obesity.
But obesity is a cause of 13 different types of cancer, and trumps smoking as a leading cause for four of these types, Cancer Research UK said.
Its analysis shows that excess weight 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 are obese, while around a third more are overweight.
One in 10 children are 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.
Michelle Mitchell, Cancer Research UK’s chief executive, said: “As smoking rates fall and obesity rates rise, we can clearly see the impact on a national health crisis when the government puts policies in place – and when it puts its head in the sand.
“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.
“Scientists have so far identified that obesity causes 13 types of cancer but the mechanisms aren’t fully understood.
“So further research is needed to find out more about the ways extra body fat can lead to cancer.”
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.”