A drug being bought online is believed to be the reason for a 40% drop in new rates of HIV in London.
Pre-exposure prophylaxis, or PrEP, reduces the risk of catching the virus and costs around £40 a month.
Four London sexual health clinics saw dramatic falls in new cases of HIV last year compared to 2015.
Dr Michael Brady, medical director of the Terrence Higgins Trust, said the drop in rates is "impressive" and "really exciting".
"I think there are probably a number of reasons, and it kind of reinforces the message that we have been saying for a while that we need to take combined approach to HIV prevention," he said.
"I don't think one single approach is going to make a major impact on the epidemic so I think what we are seeing is a mixture of the impact of better testing... earlier treatment... and increasing numbers of people accessing PrEP, which is a tablet which if taken regularly can prevent someone from getting HIV.
"These drops in diagnoses are probably the first signs we have seen in London of the impact of that combination approach."
The NHS lost a legal battle against funding PrEP in November and, as part of a clinical trial, will now be rolling out the drug to 10,000 people over three years from the summer.
The medication, also called Truvada, is used to treat people who are HIV positive but is also taken as a preventative measure in other countries.
Early data suggests it can reduce the risk of infection by up to 86%.
Its success worldwide has led many people in Britain to source generic PrEP online from places like India and China.
Sexual health clinics have also been carrying out urine and blood tests on people using the drug to monitor their health.
The use of PrEP, however, has led to some concerns it may discourage the use of condoms and cause an increase in sexually transmitted infections.
Gay rights campaigner Peter Tatchell said: "According to Public Health England in 2015, among men who have sex with men, there was a 21% increase in new gonorrhoea infections and a 19% rise in syphilis.
"That is quite a worrying because this was before PrEP became more widely available so there is an anxiety that men who are using PrEP may not use cordons and it could lead to an explosion in STIs."
Krishen Samuel, 30, is HIV positive wants the drug to be made available to those most at risk as soon as possible.
"I wish it had been available when I was HIV negative because it is an amazing tool," he said.
"Sometimes we need to face the reality most prevention campaigns speak about condoms and only focus on condoms but the reality is people do have sex without condoms as well. And if they have another tool that can prevent HIV infection, why not choose it."
In a statement given to Sky News Dr Valerie Delpech, head of HIV surveillance at Public Health England, said: "Provisional data suggests that HIV diagnoses among gay and bisexual men in England has fallen although it is not possible to confirm this at a national level until all data for 2016 have been received.
"We are closely monitoring the situation and conducting analyses of testing and other information that will provide a better understanding of the factors, including PrEP, that may be driving changes in HIV transmission among gay and bisexual men."