Obesity could overtake smoking as the most common preventable cause of cancer in the UK within 10 years, said charity Cancer Research UK.

Yet millions of people seemed unaware of the health risks from a combination of too much weight, too little exercise and high-fat diets.

A survey for the charity by pollsters NOP showed that only 29 of 1,000 people knew they were more likely to develop cancer if they were overweight. This compared poorly with their knowledge that excess weight increased the risks of heart disease, a link cited by 70 per cent or of diabetes - 16 per cent.

Lesley Walker, the charity’s director of cancer information, said: “The message for us is clear. We need to do more to get into people’s consciousness. The results of the survey are extremely worrying.”

The exact contributions of lack of exercise, diet and sheer weight are not fully understood, although eating plenty of fruit and vegetables and physical activity are known to help prevent cancer.

In the UK, it is believed that around five per cent of all new cancer cases in women, approximately 6,800 cases a year, and three per cent in men, approximately 4,000 cases could be avoided if no one was overweight or obese.