The number of new prostate cancer cases around the world will more than double over the next two decades as poorer countries catch up with the ageing of richer nations, according to a Lancet report published Thursday.

"Our findings suggest that the number of new cases annually will rise from 1.4 million in 2020 to 2.9 million by 2040," said the medical journal, based on a study of demographic changes.

Researchers behind the study said the rise in cases is linked to the increased life expectancy and changes in the age pyramid around the world.

Prostate cancer is the most widespread cancer amongst men, accounting for about 15 percent of cases. It mostly emerges after the age of 50 and becomes more frequent as men age.

As life expectancy improves in developing countries, the number of prostate cancer cases also increases, the researchers said.

They stressed that public health policies could not affect the change as they could with lung cancer or heart diseases.

Hereditary factors are much less manageable than, for example, smoking is in the cause of lung cancer. A link with weight has been established but it is not yet known if this a direct cause of prostate cancer.

Researchers also said that health authorities had to encourage earlier screening in developing countries as the disease is often diagnosed too late to give an effective treatment.