Studies have shown that the chance of developing major killers like breast and kidney cancer rises as people become increasingly overweight.
Health experts foresee that if trends continue up to one third of British women and half of men could be obese by 2050, up from around a quarter at the moment.
Professor Martin Wiseman, the medical and scientific adviser to the World Cancer Research Fund (WCRF) charity, says that today's children will face increasing rates of cancer as they grow up unless tough action is taken.
"Unless something happens soon to stop the increase in obesity then we are sleepwalking towards a situation where Britain will be facing more cancer cases than ever before," the Telegraph quoted the expert, as saying.
"The evidence now shows that, after not smoking, maintaining a healthy weight is the most important thing you can do for cancer prevention.
"This means that if rates of obesity continue to rise then this will have serious consequences for cancer rates in Britain," the expert added.
He added that he expected the number of cancer cases to double over the next 40 years.
Research has shown that becoming overweight increases the likelihood of developing six different types of cancer, including breast, kidney, womb and pancreas.