The UK government is to provide £24m to fund a trial to assess how well a microbicide gel can prevent HIV infection in women.
HIV is a massive problem among African women
Aids is the biggest killer in Africa, and most new HIV infections occur among young African women.
Research has shown an effective gel, applied before sex, could prevent up to 2.5m HIV infections world-wide over three years.
An extra £2m will be provided by the Medical Research Council.
It is thought that many women in Africa have little control over sexual relations.
Women are also biologically more susceptible to HIV infection than men.
Hilary Benn, International Development Secretary, said: "Women vulnerable to infection are frequently unable to refuse sex or to insist on the use of a condom.
"The £26m announced today will fund a final stage trial of a microbicide gel that, if effective, could help women to protect themselves against HIV infection and help reverse the spread of the disease globally."
Professor Janet Darbyshire, Director of the MRC's Clinical Trials Unit, said: "The funding will take us one step further towards identifying an effective microbicide - a crucial element of our effort to reduce HIV transmission."
The microbicide gel being trialled is called PRO2000.
The MRC's Clinical Trials Unit will test its effectiveness and safety, starting in four African countries: Uganda, South Africa, Tanzania and Zambia.
The trial is expected to begin within 4 months and will run for 39 months.
A further six months will be needed to finalise analyses and report on findings.
Figures show that 57% of the 25m adults in sub-Sahara Africa infected with HIV are women.
Microbicides work in one of three ways - by killing the virus before it enters the body; by preventing it from taking hold once inside the body; or by creating a barrier to stop it from entering the body in the first place.
PRO2000 prevents HIV from binding onto cells and infecting them.
Nick Partridge, director of the HIV charity Terrence Higgins Trust said: "The development of a microbicide would be a significant step forward in the fight against HIV, as would the discovery of an effective vaccine.
"But it is important to remember that these are long term strategies that will take many years to reach the people that need them and may never be successfully developed.
"In the short term, promoting condom use and good sex education are essential if we are to prevent more unnecessary deaths from HIV."
Michael Carter, of NAM, an HIV information service, said: "The announcement of extra government funding for research into this microbicide is extremely welcome.
"Results from earlier studies involving PRO 2000 have been encouraging, but we'll have to await the results of this larger study to know if it is a safe and effective way of preventing HIV."
Achmat Dangor, of UNAIDS, said microbicides were one of the most promising new prevention options on the horizon.
"The development of an effective microbicide would be a breakthrough in giving women a real choice when it comes to protecting themselves from HIV and prevent the further spread of the Aids epidemic."