A "game-changing" trial has shown that rates of HIV infection can be slashed by treating actively gay men with an anti-viral drug when they are healthy.

The Proud study, conducted in England, provides the first evidence that prophylactic HIV treatment is highly effective in a real-world setting.

It showed that pre-exposure to the HIV drug Truvada can reduce the risk of infection in men-who-have-sex-with-men (MSM) by as much as 86%.

Previous research had suggested that prophylactic treatment might cut HIV infection rates but it was unclear whether such an approach would work in practice.

The new study recruited 545 participants at 13 sexual health clinics in England. They were divided into two groups, one of which was to be given Truvada immediately and the other a year later.

Comparing the two made it possible to assess the effectiveness of pre-exposure prophylaxis (PrEP) in men at high risk of HIV infection.

Of the 276 men treated straight away, just three were infected with HIV over the subsequent year. During the same period, 19 of the men from the "deferred group" became HIV positive.

Chief investigator Professor Sheena McCormack, from the Medical Research Council's Clinical Trials Unit at University College London, said: "These results are extremely exciting and show PrEP is highly effective at preventing HIV infection in the real world. Concerns that PrEP would not work so well in the real world were unfounded.

"These results show there is a need for PrEP, and offer hope of reversing the epidemic among men who have sex with men in this country. The findings we've presented today are going to be invaluable in informing discussions about making PrEP available through the NHS."

Dr Michael Brady, medical director at the HIV/Aids charity Terrence Higgins Trust, said: "PrEP is, quite simply, a game-changer.

"We know that most gay men use condoms most of the time, and that this has prevented tens of thousands of HIV infections since the epidemic began in the UK. However, we also know that condomless sex vastly increases the risk of HIV being transmitted.

"This research shows just how effective PrEP can be in preventing transmission of the virus in groups at greatest risk; offering another line of defence alongside condoms and regular testing. 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.

"We urge the Government, NHS England and local authorities to make PrEP a key priority in the fight against HIV."

The findings, presented at the Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle, US, also showed that taking a drug to protect against HIV did not appear to alter rates of condom use.

No significant impact was seen on levels of infection of other sexually transmitted infections (STIs).

Deborah Gold, chief executive of the National Aids Trust, said: "The study shows two important things - one, that PrEP is highly effective in preventing HIV and two, that gay men won't just throw away the condoms as soon as they start taking PrEP (STIs were the same in men taking and not taking PrEP).

"During the study period there were 19 HIV infections in the group not taking PrEP. 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 £7 million. 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."

Proud stands for PRe-exposure Option for reducing HIV in the UK: immediate or Deferred.

In October last year, an ethical decision was taken to offer men in the "deferred group" arm of the study Truvada earlier than planned.

This followed an interim analysis of the data that revealed the extent of the protection provided by prophylactic treatment.

Sexually active gay and bisexual men are the population group at highest risk of HIV infection. In 2013, an estimated 2,800 new HIV infections occurred in MSM in the UK.

Dr Des Walsh, head of infections and immunity at the Medical Research Council, which co-funded the study, said: "HIV remains a serious public health concern - in 2013 alone, around 2,800 gay, bisexual and other men who have sex with men became infected.

"So clearly, additional approaches are needed to tackle the HIV epidemic, particularly for populations at higher risk.

"The Proud study addresses this very important issue and shows promising results that a relatively straightforward intervention based on existing therapy - PrEP - could have a major impact in preventing HIV infection."