Even people of a healthy weight should slim if they want to reduce their risk of developing cancer, a landmark study advised today.

Everyone should aim to be as slim as possible without being underweight, said the World Cancer Research Fund.

Its analysis of 40 years of international research into the links between lifestyle and cancer also advised that people should not eat processed meat, such as ham and bacon, gain no weight after 21 years of age, and avoid drinking alcohol.

The researchers found "convincing" evidence that excess body fat can cause six different types of common cancers, including those of the breast, bowel and pancreas.

They said that to enjoy maximum protection against cancer everyone should be at the lower end of the healthy weight range.

Sir Michael Marmot, who chaired the expert panel, said: "We are recommending that people aim to be as lean as possible within the healthy range, and that they avoid weight gain throughout adulthood.

"This might sound difficult, but this is what the science is telling us more clearly than ever before. The fact is that putting on weight can increase your cancer risk, even if you are still within the healthy range.

"So the best advice for cancer prevention is to avoid weight gain, and if you are already overweight then you should aim to lose weight."

People with a Body Mass Index (BMI) of 18.5 to 24.9 are 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.

But the study said the risk of cancer rises as people get nearer to a BMI of 25 and everyone should try to be as close to the lower end of the weight range as possible.

The researchers said body fat was a key factor in the development of cancer, claiming that its links to cancer are much stronger than generally realised.

The report is based on an analysis of 7,000 cancer studies published since the 1960s, and includes recommendations from a panel of 21 world-renowned scientists.

The report concludes there is strong evidence that excess body fat is a trigger for cancers of the oesophagus, pancreas, bowel, kidney, the lining of the womb, and breast cancer in post-menopausal women.

The survey also links diet to the risk of developing certain cancers, particularly those of the bowel.

The researchers particularly warned against eating processed meat, including ham, bacon, pastrami, salami, and frankfurters.

The study also found a strong link between eating red meat and colorectal cancer and recommend that people consume no more than 500g of cooked red meat a week.

All alcoholic drinks should be avoided to protect against cancer, but the study acknowledged that modest consumption was said to have a protective benefit for heart disease. Alcohol was particularly linked to mouth, oesophagus and breast cancers.

Mothers are advised to breastfeed exclusively for six months and to continue with complementary breastfeeding after that to protect against breast cancer and "probably" protect their child against obesity later in life. The most recent data shows that in 2004 nearly a quarter of men and women in England were obese, as well as 10% of girls and 8% of boys aged under 20.

A government report published earlier this month warned that if trends continue some 60% of UK men, 50% of women and a quarter of all children could be clinically obese by 2050.