Almost 800,000 cancer cases a year are caused by being overweight or diabetic, a study shows.
And women who are carrying too many pounds are almost twice as likely to develop the disease as men, according to the research.
The staggering toll underlines the dangers of the obesity epidemic - especially among children.
It’s the first time the number of cancers likely to be caused by the conditions has been calculated.
The results are particularly worrying for the UK with more than two thirds of men and almost six in ten women overweight or obese.
More than four million Britons are living with diabetes - about nine-in-ten with the type 2 form linked to obesity.
Cancers caused by diabetes and being overweight or obese were almost twice as common in women as men - accounting for 496,700 and 295,900 respectively.
Breast cancer was the commonest cancer - accounting for 147,400 cases or 29.7 per cent.
Endometrial cancer - which begins in the lining of the womb - was the second with 121,700 cases linked (24.5%).
Lead author Dr Jonathan Pearson-Stuttard, of Imperial College London’s School of Public Health, said: “While obesity has been associated with cancer for some time the link between diabetes and cancer has only been established quite recently.
“Our study shows diabetes - either on its own or combined with being overweight - is responsible for hundreds of thousands of cancer cases each year across the world.”
High insulin or glucose levels, chronic inflammation and sex hormone disruption could be behind the alarming phenomenon.
The researchers say the figures highlight the need for effective food policies to tackle overweight and diabetes.
They also called for doctors to be aware of the high cancer risk carried by people of all ages who are overweight, have diabetes or both.
The study published in The Lancet Diabetes & Endocrinology found nearly six per cent of new worldwide cancer cases in 2012 were caused by the combined effects of diabetes and being overweight (BMI of over 25) or obese (over 30).
Dr Pearson-Stuttard said: “Both clinical and public health efforts should focus on identifying effective preventive, control and screening measures to structurally alter our environment, such as increasing the availability and affordability of healthy foods, and reducing the consumption of unhealthy foods.
“It’s vital coordinated policies are implemented to tackle the shared risk factors and complications of chronic diseases such as obesity and diabetes.
“In the past, smoking was by far the major risk factor for cancer, but now healthcare professionals should also be aware that patients who have diabetes or are overweight also have an increased risk of cancer.”
Being overweight was responsible for twice as many cases with 544,300 due to a high BMI (body mass index) - equivalent to 3.9 per cent. Diabetes caused 280,100 - or two per cent.
One-in-four of the latter in 2012 (77,000) were attributable to the worldwide rise in diabetes between 1980 and 2002.
Just under a third of weight-related cancer cases in 2012 (174,000) were attributable to the worldwide rise in the number of overweight and obese people over the same period.
The researchers say if global rates of diabetes and overweight continue to rise the share of cancers due to them will increase by over 30 per cent in women and 20 per cent in men by 2035.
Almost nine-in-ten people living with type 2 diabetes are overweight or have obesity. People who are overweight or have obesity have added pressure on their body’s ability to use insulin to properly control blood sugar levels, and are therefore more likely to develop diabetes.
The study gathered data on cases of 12 types of cancer from 175 countries in 2012. They combined this with information on diabetes and high BMI and on diabetes - matching the sets by age group and gender.
The two factors accounted for a quarter of liver cancers and more than a third of all endometrial cancers worldwide.
In men, liver cancer was the commonest cancer caused by diabetes and high BMI, accounting for 126,700 cases, or 42.8 per cent of all cancers caused by diabetes and high BMI.
Bowel cancer was the second most common - accounting for 63,200 new cases or 21.4 per cent.
The most cancer cases were seen in high-income western countries (38.2 per cent, 303,000 of 792,600 cases).
But the largest increases from 1980 to 2002 were seen in low and middle-income countries.
The researchers said diabetes is associated with bowel, pancreas, liver, gallbladder, breast and endometrial cancers.
High BMI is associated with multiple myeloma, bowel, gallbladder, pancreas, kidney, liver, endometrial, breast, ovarian, stomach, thyroid and oesophageal cancers.