The "morning after pill" for HIV is given to rape victims via police referral

The two men have been granted legal aid on public interest grounds to take the government to court over the policy on post-exposure prophylaxis (PEP).

A month-long course of PEP, which uses anti-retroviral drugs, can cut the risk of infection after exposure.

One of the pair passed the virus to the other after a condom split, but they say they were unaware of the treatment.

PEP is routinely given to healthcare workers accidentally exposed to HIV at work and is made available to rape victims through police referral. To be most effective, it must be given immediately after exposure.

The problem is people do not know about it and if they did they would struggle to find a doctor who would give it to them

Frances Swaine, solicitor

It is also available on a case-by-case basis for those exposed through consensual sex, although the couple argue the public and doctors are not aware of this.

The pair, aged 35 and 26, from the south east of England, are expected to call for a major campaign to raise awareness about the treatment.

Their solicitor, Frances Swaine, of Leigh, Day and Co, said: "The case is being brought to raise awareness about PEP.

"The problem is people do not know about it and if they did they would struggle to find a doctor who would give it to them.

"What we are saying is that there needs to be guidelines on PEP and a campaign to tell people about it. That is not to say everyone should have access to it, it would be up to clinical judgement to decide that."

The drugs are extremely toxic and the side-effects can include nausea, headache, diarrhoea and vomiting, but Ms Swaine said most people would prefer to put up with that rather than risk infection with HIV.

The Department of Health said there was little evidence that PEP was effective in cases other than "needle-stick" injuries to health care workers, but individual doctors could still prescribe the drugs.

A spokeswoman added: "PEP is not a substitute for safer sex and condom use continues to be the main HIV prevention message."

The case is likely to be heard at the High Court in the new year.