A leading Aids charity has won a High Court battle over whether a preventative treatment for HIV which charities say is a "game-changer" can legally be funded by the NHS.
NHS England said it had received advice that it does not have the legal power to fund pre-exposure prophylaxis (PrEP), a "highly effective" anti-retroviral drug used to stop HIV from becoming established in the event of transmission.
But Mr Justice Green, sitting in London, ruled that NHS England "has erred in deciding that it has no power or duty to commission the preventative drugs in issue".
The ruling was a victory for the National Aids Trust (NAT), which brought the case to court.
When taken consistently, PrEP has been shown to reduce the risk of HIV infection in people who are at high risk by more than 90%.
A row erupted earlier this year after NHS England said it would not routinely fund the drug.
In March, the body decided the treatment was a preventative service and was therefore not its responsibility. It has said local councils are in charge of funding preventative health services.
However, NHS England agreed to a re-evaluation after the NAT launched a legal challenge.
Then in May it said it had "considered and accepted NHS England's external legal advice that it does not have the legal power to commission PrEP", and that under 2013 regulations "local authorities are the responsible commissioner for HIV prevention services".
Allowing NAT's application for judicial review, Mr Justice Green said on Tuesday the core of the legal challenge was about "the allocation of budgetary responsibility in the health field".
He said: "No one doubts that preventative medicine makes powerful sense.
"But one governmental body says it has no power to provide the service and local authorities say they have no money.
"The claimant is caught between the two and the potential victims of this disagreement are those who will contract HIV/Aids but who would not were the preventative policy to be fully implemented..
"In my judgment the answer to this conundrum is that NHS England has erred in deciding that it has no power to commission the preventative drugs in issue."
Alternatively, said the judge, NHS England has "mischaracterised the PrEP treatment as preventative when in law it is capable of amounting to treatment for a person with infection".
In any event NHS England had power to commission preventative treatments because that facilitated, or was incidental to, "the discharge of its broader statutory functions".
Campaigners have said that while the majority of gay men use condoms to prevent being infected with HIV, there is also an "ethical duty" to provide PrEP to those who do not.
And they said the drug would provide an additional defence against HIV - and would not be used simply as an alternative to safe sex.
It comes after the results of a trial, published in February 2015, suggested that rates of HIV infection could be slashed by treating actively gay men with the anti-viral drug when they are healthy.
Dr Michael Brady, medical director at the HIV/Aids charity Terrence Higgins Trust, welcomed the findings and described the drug as "a game-changer".
He said PrEP offered "another line of defence" against HIV, alongside condoms and regular testing.
Dr Brady said: "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 judge gave NHS England permission to appeal against his ruling at the Court of Appeal.
Dr Jonathan Fielden, NHS England's director of specialised commissioning and deputy national medical director, said the appeal would be against the conclusions reached by the judge as to the scope of NHS England's legal powers under the National Health Service Act 2006.
Dr Fielden added: "In parallel with that we will set the ball rolling on consulting on PrEP so as to enable it to be assessed as part of the prioritisation round.
"Of course, this does not imply that PrEP - at what could be a cost of £10-20 million a year - would actually succeed as a candidate for funding when ranked against other interventions.
"But in those circumstances, Gilead - the pharmaceutical company marketing the PrEP drug Truvada - will be asked to submit better prices, which would clearly affect the likelihood that their drug could be commissioned."
The Local Government Association (LGA) welcomed the court's ruling.
The LGA's community well-being portfolio holder, Izzi Seccombe, said: "We are pleased that today's ruling confirms our position that NHS England has the power to commission the HIV treatment PrEP.
"During the transition period to the implementation of the NHS and Care Act 2010, NHS England sought to retain commissioning of HIV therapeutics, which the PrEP treatment clearly falls into.
"We therefore believe that it is, and should remain, an NHS responsibility.
"Councils have invested millions in providing sexual health services since taking over responsibility for public health three years ago, and we look forward to continuing to work with NHS England to determine how PrEP could be introduced across the country, and reduce the risk of HIV infection.
"We hope NHS England now works without further delay to bring this matter forward to decide whether PrEP should be commissioned."
Copyright (c) Press Association Ltd. 2016, All Rights Reserved. Picture - Yusef Azad, Director of Strategy National Aids Trust, outside the Royal Courts of Justice, The Strand, London after the leading Aids charity has won a High Court battle over whether a preventative treatment for HIV which charities say is a "game-changer" can legally be funded by the NHS (c) Jonathan Brady / PA Wire.