A decision is due in the Court of Appeal over who pays the bill for a service to block HIV infection.
The High Court ruled that NHS England has the power to commission PrEP, which has been described as a "game changer" in the fight against HIV/Aids.
NHS England is appealing on the grounds that local authorities have the legal responsibility to arrange services to "prevent" the spread of HIV, while its own responsibilities are limited to treating those already assumed to be infected.
PrEP, short for "pre-exposure prophylaxis", is a prevention strategy which involves people who are HIV negative but at high risk of infection taking the anti-retroviral drug Truvada to reach optimal levels of protection.
The anticipated cost of providing PrEP services is £10m-£20m a year.
When taken consistently, it has been shown to reduce the risk of HIV infection in people who are at high risk by more than 90%.
The legal battle is important and urgent because of its potential impact on the provision of other services, including hearing implants for children with deficient or missing auditory nerves, prosthetics for lower limb loss, and a drug for treating certain mutations in children aged two to five with cystic fibrosis.
Nine new treatments and services NHS England had planned to make available to patients have been put on hold pending the decision on Thursday.
In May this year, NHS England's specialist services commissioning committee decided not to commission PrEP, saying it lacked power to do so under NHS legislation and regulations.
But Mr Justice Green said that it did have the power under the NHS Act 2006 and under regulations made in 2012 which came into effect in April 2013.
The ruling was a victory for leading charity the National Aids Trust (NAT) with backing from the Local Government Association (LGA).
Jonathan Swift QC, appearing for NHS England, has asked Lord Justice Longmore, Lord Justice Underhill and Lady Justice King to rule the High Court decision wrong in law and to conclude that provision of PrEP services does indeed fall outside NHS England's legal powers.