Male circumcision could decrease the rate of infection by HIV in Africa by as much as 37%, a study has concluded.

Scientists used data from a previous clinical trial to predict the impact of male circumcision on rates of HIV infection in Africa in the study, which was published in the Public Library of Science journal.

It said making male circumcision compulsory in areas of sub-Saharan could avoid up to 6m new cases of HIV infection and save three million lives over the next 20 years.

However, researchers warned that more information was needed on the safety and cost of circumcision practices and the feasibility of making circumcision available to all men in sub-Saharan Africa.

They said education would also be needed to ensure circumcised men did not stop using condoms because of a belief they were protected against the virus.

The reasons why male circumcision offers protection against HIV infection is not well understood. It is thought that after circumcision the glans of the penis gets harder, making it more difficult for the virus to penetrate. The foreskin also contains Langerhans cells, one of the major binding sites for the virus.

Dr Brian G Williams, an epidemiologist at the World Health Organisation, said: "In the case of HIV, from one infected person, between five and seven additional people will become HIV positive. Male circumcision may be able to reduce this figure by about a half.

"We then need to reduce the remaining number through increased condom usage, anti retroviral therapy and treatment of other sexually transmitted diseases.

"Male circumcision can have an impact on HIV infection. On its own it is not enough to eliminate infection, but, when combined with other methods, it may lead to eventual elimination of HIV."

The full benefits of circumcision would only be apparent between 10 and 15 years after a programme was started - the average length of time it takes for the virus to progress - and it could not be seen as a replacement for conventional HIV/Aids treatments, the study said.

"The good news is that male circumcision can have a massive impact, but we won't see its benefit for at least 15 years. Whatever we do, this is a long term [approach]" said Dr Williams.

Africa has the highest incidence of HIV/Aids in the world, with UN statistics showing that up to 34% of adults in South Africa are HIV positive.