The National Aids Trust (NAT) says it will challenge the NHS in court over a decision about the provision of a drug treatment called Prep that can prevent HIV.
NHS England says it has repeatedly looked at the issue and will not fund the “game-changer” treatment.
It says the responsibility lies with local authorities.
The NAT says it will seek a judicial review unless NHS England changes its mind.
Prep (pre-exposure prophylaxis) is a daily pill that can considerably lower a person’s chances of catching HIV.
If taken correctly, it is almost entirely effective in preventing HIV.
NHS England has promised £2m in funding to treat about 500 people with Prep over the next two years.
But after considering representations from stakeholders, it concluded that it was not the body responsible for offering the drug.
According to NHS England, local authorities are the “responsible commissioner” for HIV prevention services.
In a letter published in the British Medical Journal, NAT chief executive Deborah Gold said: “Faced with this impasse, NAT has no choice but to take the matter before a court for judicial review.”