Elton John praises late friend Princess Diana ahead of candid documentary about her life

The Candle in the Wind singer spoke about his friend Diana in a new ITV documentary

Elton John has shared a sweet picture of himself with Princess Diana ahead of Monday's candid documentary, Diana, Our Mother: Her Life and Legacy. The 70-year-old, a close friend of the late princess, will join Prince William and Prince Harry in the one-off special, which celebrates the life and work of the Princess twenty years after her death. Alongside the photo, Elton wrote: "I share my memories of Princess Diana and her vital charity work tonight on #DianaOurMother, 9pm on @itv. As an AIDS activist, she changed so much for so many and will never be forgotten."

WATCH: Clips of Princes William and Harry remembering their mum Diana

In a teaser from the show, Elton discussed the late royal's incredible work as an AIDS activist. He recalled the moment she shook hands with a patient at Britain's first Aids unit at London's Middlesex Hospital in 1987. Sir Elton said: "It was considered to be a gay disease and for someone who was within the Royal Family and who was a woman, and who was straight, to have someone care from the other side, was an incredible gift."

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Taking a look through some archives of Diana's pictures, Elton added: "Look at her face. In every photo there's a positive, wonderful smile. Also she had that incredible ability, which he kind of inherited - and I told him that, and he said thanks very much - to make people feel at ease and make them feel that everything's going to be all right. 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." Following in his mother's footsteps, Harry has become a prominent HIV and AIDS campaigner through his charity Sentebale set up in 2006 to help children in Africa.

Buckingham Palace opens for summer with a tribute to Princess Diana: all the details

Don't miss HELLO!'s ultimate souvenir edition celebrating the Queen of Hearts, on sale now. Look back at her life through 148 pages of beautiful images and revealing interviews with those who knew Diana best. Her fashion, her inspirational charity work and love for the sons who now carry her flame forward - it's all there in this unmissable HELLO! special.