The National Aids Trust, a leading charity, brought the case against NHS England after the health service said it had received external legal advice that it did not have the power to fund pre-exposure prophylaxis (PrEP).

The drug, described by the trust as a “game-changer”, is an antiretroviral used to stop HIV from becoming established in the event of transmission. It is aimed particularly at gay men who engage in high-risk sex without using a condom.

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 per cent.

NHS England had said decisions about whether to fund the drug must be taken by local councils, which in 2013 assumed responsibility for preventive health. However, local authorities argued they lacked the money to pay for it.

Mr Justice Green, sitting in London, said the “potential victims” of the disagreement “are those who will contract HIV/Aids, but who would not were the preventative policy to be fully implemented”.

He suggested that NHS England had mischaracterised the PrEP treatment as preventive when in law it was capable of amounting to treatment for an infected person.

In any event, he said, the NHS had the power to commission preventive treatments that facilitated, or were incidental to “the discharge of its broader statutory functions”.

Jonathan Fielden, NHS England’s director of specialised commissioning, said its lawyers had advised it to appeal against the conclusions reached by the judge “as to the scope of NHS England’s legal powers under the National Health Service Act 2006”.

Meanwhile, it would “set the ball rolling” so that PrEP could be assessed as part of the “prioritisation round”, which determines how money should be allocated. This, Dr Fielden said, did not imply that PrEP — at what could be a cost of £10m-£20m a year — would actually succeed as a candidate for funding when ranked against other potential interventions.

As part of that process, Gilead, the pharmaceutical company marketing the PrEP drugTruvada, would be asked to submit better prices, “which would clearly affect the likelihood that their drug could be commissioned”, he added.

Michael Brady, medical director at the Terrence Higgins Trust, described the drug as “a game-changer” that offered “another line of defence” against HIV, alongside condoms and regular testing.

Speaking for the Local Government Association, Izzi Seccombe welcomed the ruling and said councils had “invested millions in providing sexual health services since taking over responsibility for public health three years ago”.

Ms Seccombe added: “We hope NHS England now works without further delay to bring this matter forward to decide whether PrEP should be commissioned.”-Financial Times