Further evidence that taking anti-HIV drugs stops gay men passing on the virus to sexual partners has been called a "powerful message" which should be more widely known.
A study of nearly 1,000 gay male couples in The Lancet found no cases of HIV transmission over eight years.
This was due to treatment reducing the virus to very low levels in the body.
"Undetectable equals untransmittable" should be basic HIV knowledge for everyone, experts said.
The European study followed 972 gay male couples - where one was living with HIV and taking antiretroviral therapy (ART) and the other was HIV negative - over eight years, from 2010-2017.
There were no cases of HIV being passed within the couples over that time.
And the researchers say that around 472 cases of HIV are likely to have been prevented.
In total, the couples reported having anal sex without condoms a total of 76,088 times.
Although 15 men did become infected with HIV during the study, genetic testing showed that none of the viruses came from their main partner.
"Our findings provide conclusive evidence that the risk of HIV transmission through anal sex when HIV viral load is suppressed is effectively zero," the researchers said.
Prof Alison Rodger, study author and professor of infectious diseases at University College London, said anal sex was known to have the highest risk of transmission, but gay men should be reassured.
"This powerful message can help end the HIV pandemic by preventing HIV transmission, and tackling the stigma and discrimination that many people with HIV face."
She called for all people living with HIV to have access to testing and effective treatment.
'Huge relief to know I can't pass on virus'
Matt Stokes, 26, was diagnosed with HIV in 2016 and started drug therapy four weeks later. Tests showed the virus was undetectable in his body within three months.
"It's a huge relief and gives me self confidence to know I can't pass on the virus," he says.
"Among the gay community and my friends there has been a real change in recent years - you can put 'undetectable' on Grindr, for example."
But there's a long way to go before everyone knows what it means, he adds.
"Some people don't want to believe it. They have an irrational fear it might not be true."
He says charities campaigning on the issue are giving the message "a huge boost" and changing views on sex.
Deborah Gold, chief executive of NAT (National AIDS Trust) said more should be done to get the message out to healthcare workers and the general public.
"There needs to be a much better understanding of how HIV is and isn't transmitted, and the fact that treatment stops transmission, in the NHS and beyond. We think this is vital to addressing stigma."
Previous research has shown zero risk for heterosexual couples of passing on the virus, when one person is taking HIV treatment, prompting UNAIDS to launch its undetectable = untransmittable campaign.
Dr Ford Hickson, from the London School of Hygiene and Tropical Medicine, said the study confirmed that if people suppress their HIV with antiretroviral therapy, they "cannot pass their virus to other people during sex, whatever kind of sex they have".
In the study, the men with HIV had been taking antiretroviral therapy for an average of four years before it began, making the virus undetectable - defined as fewer than 200 copies per ml of blood.
Most people reach this level after taking daily HIV treatment for six months.
What is antiretroviral therapy?
It is a combination of drugs, to be taken daily, to stop HIV replicating in the body.
It can't cure HIV, but it can reduce the amount of virus to undetectable levels in the blood.
Most people with HIV take a combination pill once a day but others can take up to four pills a day depending on their specific health needs.
Everyone is recommended to start treatment straight away after being diagnosed.
Dr Michael Brady, medical director at Terrence Higgins Trust, said: "The study has given us the confidence to say, without doubt, that people living with HIV who are on effective treatment cannot pass the virus on to their sexual partners.
"This has incredible impact on the lives of people living with HIV and is a powerful message to address HIV-related stigma."
In the UK, 98% of people diagnosed with HIV are receiving treatment - and 97% of those have an undetectable level of the virus - meaning they are unable to pass on the infection.
But figures from Public Health England estimate than only 92% of the 101,600 of people living with HIV have been diagnosed.