It found that over the past four decades, the average age-corrected male BMI rose to 24.2 from 21.7, and in women rose to 24.4 from 22.1.

London - Obesity will cause almost 700 000 cases of cancer within the next 20 years, experts warn.

They predict that four in ten adults will be obese by 2035. Estimates also show that by this time obesity-related illnesses, including diabetes and heart disease, will cost the NHS more than £7.5-billion a year.

A report by Cancer Research UK and a group of other health charities urges the UK government to take far tougher action. They recommend banning junk food adverts between 6am and 9pm so they are less likely to be seen by children.

They also want a 20 per litre tax on sugary drinks. But the government has repeatedly dismissed introducing a sugar tax, despite similar pleas from health bodies and MPs.

Ministers are due to publish their long-awaited obesity strategy amid concerns from experts that they are not addressing the crisis.

Currently, around 30 percent of adults – almost one in three – are classed as overweight and a similar number are obese, meaning a total of around 60 percent are deemed too fat.

By 2035, this is predicted to rise to 73 percent of adults, including 41 percent who are obese. The worst-affected group will be women on the lowest incomes, almost half of whom will be obese, according to the report.

The researchers used computer modelling to estimate that this will lead to 670 000 new cancer cases. Obesity is known to increase the risk of many types of cancer including bowel, breast, prostate, pancreatic and ovarian cancers.

Alison Cox, director of cancer prevention at Cancer Research UK, said: “Obesity will be a huge burden to society and the NHS in the near future. We must act now to combat this threat.

“Kids are bombarded with advertisements for unhealthy food. It’s vital the Government restricts this kind of advertising ... Otherwise our children will pay the price and will have poorer health, face more disease and die earlier.”

Paul Lincoln, chief executive officer at the UK Health Forum, which represents almost 100 charities and medical bodies, said: “This report makes a very clear economic case for why we must act now to turn the rising tide of obesity.”