Their study, on 545 high-risk men, found one case of HIV could be stopped for every 13 men treated for a year.
The research team says it would be similar to the pill for women and would not encourage risky sex.
The findings have been described as a "game changer" and the NHS is considering how to adopt them.
Antiretroviral drugs have transformed HIV treatment and patients have a near-normal life expectancy.
Now there is a growing body of research showing the drugs can have a dramatic role in preventing new infections.
Massive fall
Gay men face a high risk of contracting HIV. In London, one in eight gay men has HIV while the figure is one in 26 in the rest of the UK.
In the first year of the study, 19 people developed HIV out of the 269 men who were not given the medicine.
There were just two cases in the 276 patients given preventative drugs - a fall of 86%.
The men were given Truvada, which has already been approved as a preventative drug in the US
The trial was altered as the early results were so promising, and all participants are now getting the drugs.
Risky sex?
Concerns had been raised that men given the drug would adopt riskier behaviours including stopping using condoms.
But the scientists found no difference in levels of other sexually transmitted infections, such as chlamydia.
"We certainly think the NHS should be considering making this available," said one of the researchers Dr Anthony Nardone.
He added: "I don't envisage all men taking PrEP [pre-exposure prophylaxis] for all their lives, but in effect what we're doing is giving men an option to get through periods of very high risk in their lives."
Fellow scientist Dr Mitzy Gafos said many gay men would not need the drugs as they were not having unprotected sex.
Estimates suggest that between 5,000 and 15,000 men in the UK would be suitable.
Dr Gafos added: "There's very clearly a group of individuals who would benefit from the availability of this product.
"PrEP is having an important impact on removing the inevitability of HIV for many individuals and enhancing the sexual experience, reducing their fears and the concerns that they go through in relationships."
The study has been presented at the Conference on Retroviruses and Opportunistic Infections in Seattle, but the full data has not yet been published in a medical journal.
HIV fear
Charlie took part in the trial because of the high risk of HIV in London
Charlie Witzel, a 27-year-old Canadian living in London, took part in the trial.
He said higher rates of HIV in the capital meant he felt he was at high risk "just by being in London".
He told the BBC: "Like a lot of gay men my age, sex has always been associated with HIV for me, that has presented various barriers with intimacy."
Mr Witzel said the knowledge that the person most likely to infect you was a regular partner was a "massive challenge" in relationships.
He thinks the drugs should be available "like the contraceptive pill" as people were "quite good at evaluating their own risk and knowing when something is not relevant" such as after entering a monogamous relationship.
The cost of the medicines would come to £360 per month per person. However, the National Aids Trust said they would pay for themselves because of the costs of treating HIV.
Chief executive Deborah Gold said: "If we can stop people getting HIV by giving them PrEP, we have an ethical duty to do so.
"Furthermore, over the course of their lifetime the treatment of those 19 men will cost the NHS nearly £7m, so the financial argument is clear, as is the ethical one.
"PrEP needs to be available on the NHS as soon as possible for all those who need it."
'Game-changer'
The Terrence Higgins Trust charity said condom use had already prevented tens of thousands of HIV infections since the 1980s, but argued PrEP would be a valuable extra weapon in the armoury.
Its medical director Dr Michael Brady said: "PrEP is, quite simply, a game-changer.
"It is not a vaccine and it won't be for everyone, but once approved, we expect it to significantly increase the momentum in our fight against the virus."
The NHS is already considering how PrEP could be introduced.
Prof Simon Barton, from NHS England, said: "The findings of this study are very important and significantly add new data to existing international evidence.
"Several questions still need to be resolved about how the greatest benefit can be delivered to those at risk of infection and how the key elements in the study, such as follow-up testing and adherence support, can be commissioned to benefit individual and public health in real life settings."
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