Prince Harry has paid tribute to his late mother's work in raising awareness of HIV and Aids, and urged more people to "embrace regular testing".
The prince was speaking at the Attitude magazine awards, where Princess Diana was posthumously honoured with the Legacy award, 20 years after her death.
Prince Harry said if his mother were still alive, she would be "standing alongside" those living with HIV.
The prince collected the award on Thursday night on her behalf.
In April 1987, Princess Diana opened the UK's first purpose built HIV/Aids unit that exclusively cared for patients infected with the virus, at London Middlesex Hospital.
In front of the world's media, she shook the hand of a man suffering with the illness.
She did so without gloves, publicly challenging the stigma and notion that HIV/Aids was passed from person to person by touch.
"She knew that Aids was one of the things that many wanted to ignore and seemed like a hopeless challenge," he told the awards ceremony in London.
"She knew that the misunderstanding of this relatively new disease was creating a dangerous situation when mixed with homophobia.
"So, when, that April [in 1987], she took the hand of a 32-year-old man with HIV, in front of the cameras, she knew exactly what she was doing."
The prince said he and his brother, the Duke of Cambridge, were "incredibly proud of what our mother achieved".
If she were still alive, he said she "would be demanding" free and available testing and treatment for people all across the world.
"I believe that she would be telling everyone across society - not just those most at risk - that with effective treatment being free and available in the UK, that we must all embrace regular testing - both for our own sake and for those that we love," he added.
As he accepted the award, the publication unveiled its new, limited edition magazine cover featuring a black-and-white photograph of Diana by Patrick Demarchelier.
