Sir Elton John pays tribute to his late friend Princess Diana

Sir Elton John has paid tribute to Princess Diana, in a new documentary, marking 20 years since the tragic car crash which cost her her life.

'Diana: Our Mother' aired on ITV last night and will also air on HBO in the United States.

Sir Elton famously sang at Diana's funeral and was a close friend of the Princess, who left behind her two sons Prince Harry and Prince William.

In the documentary, her humanitarian work abroad is highlighted, including her work around HIV and AIDS, fighting the stigma surrounding the disease.

Sir Elton says Diana changed attitudes towards the disease across the Globe. He also observed how much Prince Harry resembles his late mother:

"She had that incredible ability - which Harry kind of inherited to make people feel at ease and make them feel that everything's going to be alright".

"I haven't experienced many people in my life who have that ability, but she could walk into a room of people and make them feel as if everything was great".

Prince Harry recently led a campaign to promote sexual health testing, and even took an HIV test live on Facebook.

Harry has really continued his mother's work in this area, attending the International AIDS Conference in South Africa and visiting testing clinics.

At the time the 32 year old said: "If you respect what my mother stood for, go and get tested for her".

"If she were still here today, she would probably get tested every month, just to prove a point", he added.

It should be noted that 1 in 5 people living with HIV are unaware of their status, as the virus doesn't always show symptoms, and when it does, some just think it's a bad flu, or other virus.

The Prince also believes it's important to tackle the misconception that HIV is exclusively a "gay only" disease.

If you're in the U.K. you can watch the documentary on demand on the ITV Player.