Andrew Morton has dubbed the late Princess Diana as a "humanitarian who truly cared".

Princess Diana

Princess Diana

The 63-year-old journalist and author is known for writing the controversial biography 'Diana: Her True Story' in 1992 which explored her version of royal life with her then husband Prince Charles, and has now praised the blonde beauty as "unique".

He said: "Diana was a unique blend of humanitarian who truly cared, and global celebrity who was able to dust her charitable causes with stardust."

Andrew also spoke about the late Princess' "self-deprecating sense of humour", and said Diana - who was fatally injured in a car crash in France in 1997 - often struggled with royal life and made "desperate cries for help" during the first years of her marriage.

He added: "One of her most endearing qualities was her self-deprecating sense of humour. I remember when she was secretly assisting me with her biography and she told me about the half-hearted suicide attempt - or rather desperate cries for help - which she had made during what she called 'the dark ages' - the early part of her royal life where she felt lost and alone."

And the writer reflected on one particular moment when he asked Princess Diana - who was the mother of Prince William, 34, and Prince Harry, 32 - about her "sad incidents".

He told Hello magazine ahead of the 20th anniversary of Diana's death this August: "For the biography I was asking her questions via an intermediary [Diana would then tape her answers] and I gave him a long list of questions about these sad incidents. She took one look at them and quipped: 'He's pretty well written my obituary, hasn't he?'"