The study by the World Cancer Research Fund says even those who are a healthy weight should slim if they want to reduce their risk of developing cancer.

The landmark study examined the links between lifestyle and cancer and found "convincing" evidence that excess body fat could cause six different types of common cancers, including breast, bowel and pancreatic cancer.

Everyone is advised to stay as slim as possible without being underweight and avoid putting on the kilos after 21 years of age.

Also to be avoided are processed meats such as ham and bacon and alcohol.

The researchers say in order to achieve maximum protection against cancer, everyone should aim to be at the lower end of the healthy weight range.

The expert panel say though this might appear to be a difficult task the science clearly demonstrates that putting on weight can increase a person's cancer risk, even if they are still within the healthy range and the best advice for cancer protection was to avoid weight gain.

Experts say that a Body Mass Index (BMI) of 18.5 to 24.9 is considered to be within a healthy weight range; those with a BMI of 25 to 29.9 are considered overweight, while anyone with a 30+ rating is classified as clinically obese.

The study however has revealed that as people approached a BMI of 25 the risk of cancer rose and everyone should try to be as close to the lower end of the weight range as possible.

The researchers say body fat is a key factor in the development of cancer and its links to cancer were much stronger than was generally realised.

The study which is based on an analysis of 7,000 cancer research papers published since the 1960s also includes recommendations from a panel of 21 world-renowned scientists.

The analysis has also linked diet to the risk of developing certain cancers, particularly those of the bowel and warns people to avoid eating processed meat.

A strong link between eating red meat and colorectal cancer was also found and the scientists recommend that people consume no more than 500g of cooked red meat each week.

The scientists also recommend alcohol to be avoided to protect against cancer, but acknowledge that modest consumption has been found to have a protective benefit for heart disease; alcohol was particularly linked to mouth, oesophagus and breast cancers.

Experts say after smoking, obesity is the biggest risk factor for cancer.

They say exercise is important and people should be physically active as part of everyday life.

In the U.S. 67% of adults and a third of all children are deemed overweight or obese, while in Australia 63% of men and 47% of women are overweight or obese.

Brits too are being warned that by 2050 the way things are going 60% of men, 50% of women will be obese or overweight and the most worrying trend in all three countries is the rise in childhood obesity.

Most of the recommendations tie in with long standing advice from health experts, governments and the World Health Organization, that diets based on fruits, vegetables and whole grains, that are low in red meats, dairy products and fats, offer protection against heart disease, diabetes and cancer.

Interestingly a new survey by the American Institute for Cancer Research of 1,000 U.S. adults has also revealed some surprises and highlighted a few popular misconceptions.

Many were unaware of the link between cured and processed meats and cancer, only 49% knew that diets low in fruits and vegetables raised the risk of cancer and only 46% knew that obesity was a well-documented risk.

But 71% thought that pesticide residue on produce was a cause (that has never been shown), 56% thought stress causes cancer (not proven) and 49% believed hormones in beef cause cancer.

The experts say people appear to prefer to blame cancer on factors over which they have no control, and often on things for which no proven links to the disease exists.

The report was released by the World Cancer Research Fund and the American Institute for Cancer Research, and represents five years of study by nine teams of scientists.