For the first time, new diagnoses of HIV have fallen among men who have sex with men in England, according to data from Public Health England.
They have decreased from 2,060 in 2014-15 to 1,700 in 2015-16, while in London there was an even steeper drop.
PHE said increased testing, fast treatment with HIV therapy and the use of preventative drug Prep have all contributed to the trend.
New HIV diagnoses among heterosexuals have remained stable.
While huge advances have been made in treating HIV/Aids, there has been frustration at a lack of progress in reducing the number of men being diagnosed each year.
New infections in the UK had been stuck at about 3,000 every year in the five years up to 2015.
Valerie Delpech, head of HIV surveillance for Public Health England, said: "What we are seeing is the first downturn of the HIV epidemic in gay men."
She added: "There is absolutely no reason why we cannot scale that up to further reduce new infections in gay men - and also in all people who may be at risk of HIV in the UK, regardless of gender, ethnicity or sexuality."
Published in the journal Eurosurveillance, the PHE study looked at data from 200 sexual health clinics in England.
It found that new diagnoses of HIV in gay or bisexual men at five busy London clinics, where Prep is being trialled, had decreased from 880 in 2014-15 to 595 in 2015-16 - a drop of 32%.
And this drop had occurred despite the number of these men being tested in these clinics rising by 50% over the same period.
At 30 other London clinics, new diagnoses fell by just 8%.
And in 191 clinics across England, the drop was about 5%.
Overall, this amounted to a 17% fall in England.
Among heterosexuals, who are not included in the Prep trial, new HIV diagnoses did not fall - staying at about 1,500.
Dr Michael Brady, medical director of Terrence Higgins Trust, said after years of high rates of HIV diagnoses among gay and bisexual men, such a sharp drop in England was "remarkable".
"This points towards what can be achieved when we utilise all the weapons in our arsenal against HIV transmission," he said.
"This includes access to condoms, testing, Prep and diagnosing and treating people as early as possible so they can become uninfectious."
However, he said, there was no place for complacency.
In England, eight clinics in London and several outside the capital have taken part in a trial of the so-called "game-changer" drug Prep, which is taken by HIV negative people before sex to reduce the chance of getting HIV.
Scotland has announced it will make Prep available on the NHS to people at risk of HIV . And in Wales, the government has decided to start trialling the drug.
But Dr Brady said the drug should be available to all those at risk, as soon as possible, regardless of where they lived.
Deborah Gold, chief executive of the National Aids Trust, said: "These are incredibly exciting times for HIV.
"We are on the precipice of an absolute step-change in HIV prevention."
However, she said, further investment in HIV prevention was under threat.
"Without proper investment in prevention, HIV will remain a significant health inequality."