Obesity rates have soared in recent years
Almost 19,000 people a year in the UK are diagnosed with cancer that could have been prevented if they were a healthy weight, an expert says.
Professor Martin Wiseman said if everyone had a body mass index (BMI) under 25 it would make a big dent in the incidence of cancer.
Cancers of the breast, bowel, kidney, pancreas, oesophagus and womb lining are among those linked to obesity.
Professor Wiseman is an adviser for the World Cancer Research Fund (WCRF).
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
The WCRF recommends that people try to be as lean as possible without becoming underweight.
A healthy BMI is defined as between 18.5 and 25, but, for example, scientists estimate that someone at the top of the healthy weight range is 15% more likely to develop bowel cancer than someone at the bottom of it.
Overall, scientists estimate that about a third of the most common cancers could be prevented if people ate healthily, were physically active and maintained a healthy weight.
Speaking ahead of Cancer Prevention Week, Professor Wiseman said: "The stark fact is that every year in this country, many thousands of people are diagnosed with cancer that could have been prevented if they had maintained a healthy weight.
"This is a real cause for concern and the problem is only going to get worse if the number of people who are overweight continues to rise.
"The evidence that being overweight puts you at increased risk of cancer is stronger now than ever before and we now say that, after not smoking, maintaining a healthy weight is the most important thing you can do for cancer prevention.
"This is the reason we recommend people aim to be as lean as possible without becoming underweight."
However, Professor Wiseman said a recent survey had showed that almost 40% still did not know that excess body fat was a cause of cancer.
"This means we need to do more work to spread the message that maintaining a healthy weight is something positive people can to reduce their risk of developing cancer later in life."
Dr Ian Campbell, medical director of the charity Weight Concern, said: "The message has to be that being overweight increases the risk of some forms of cancer but losing weight, even as little as 5kg can significantly reduce the risk.
"Getting BMI down to below 25 is ideal but not achievable by everyone. But every pound lost counts towards decreasing risk and is worth the effort."
Tam Fry, of the National Obesity Forum, agreed that keeping BMI down to 25 would have a big impact on cancer.
Mr Fry said the only way to tackle the problem was to monitor children much more closely than was currently done, and to take action if they showed signs of a weight problem.
He said: "We have got to prevent people becoming overweight in the first place, once the milk has been spilt it is impossible to clean up the mess."
The WCRF also recommends that people limit their consumption of energy dense foods, and avoid sugary drinks.