The royal's comments came as he attended the Attitude magazine awards on Thursday night, where Diana, Princess of Wales was posthumously honoured with the Legacy award 20 years after her death. Prince Harry made a heartfelt tribute to his mother in setting up the charity "Sentebale", which means "forget me not", to help the vulnerable children in southern Africa struggling with HIV and AIDS.
Stepping onto the stage to applause from the unsuspecting audience and Adam & The Ants hit Prince Charming, he said: "I often wonder about what she would be doing to continue the fight against HIV and Aids if she were still with us today".
"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 United Kingdom, that we must all embrace regular testing - both for our own sake and for those that we love".
If she were still alive, he said she "would be demanding" free and available testing and treatment for people all across the world.
Prince Harry said if his mother were still alive, she would be "standing alongside" those living with HIV.
The prince said he and his brother, the Duke of Cambridge, were "incredibly proud of what our mother achieved".
"Very few individuals had the power to change the mindset of millions of people at that time, but Diana knew that she was one of them - and she chose to wield her power to improve the lives of gay men suffering with HIV/Aids". "She knew that the misunderstanding of this relatively new disease was creating a unsafe situation when mixed with homophobia".
"In April 1987, my mother was only 25 years old", he began.
The first anti-retroviral drugs were developed the year before her death in 1997.
Attitude unveiled a limited-edition cover with a photo of Diana by Patrick Demarchelier.