The designer said: "She would say 'I keep being told not to wear it, but I love this dress, my caring dress'. Of course, Diana was the first member of the Royal family to break all the rules."
Sassoon, who has spent 50 years dressing some of the world's most beautiful women at his couture house Bellville Sassoon, designed the dress for a foreign tour in 1988. The Princess went on to wear it in Lagos in Nigeria, in Sao Paulo, Brazil, and also while visiting a London Aids hospice.
Press reports in the 1990's speculated that Diana loved the dress because of its colour, but Sassoon explained: "She happened to wear that dress when visiting a hospital, and children seemed to clamour round and like it. If you are like the Princess of Wales, who loved children, you don't want a strictly formal suit for a hospital visit. You pick a very informal dress with bright colours, which that dress was. The reaction is one of awe from young children."
Princess Diana wearing her 'caring dress' in 1990
Sassoon, who made more than 70 outfits for the Princess, revealed how the Princess never took clothes too seriously despite the extent to which her appearance was scrutinised. She could even pop into his London shop and choose an outfit just hours before wearing it at a formal engagement. "She just appeared in the shop saying 'help'. I have never enjoyed such an informal relationship with a member of the Royal family, and I am sure I never will again. Princess Diana was much more relaxed about dressing, but she always got it right, she had the figure and the sheer charm," said the designer.
Where other Royal family members would earnestly practice curtsying in their new clothes, the Princess would delight in removing her thermal underwear before trying on a designer dress. Sassoon said: "She was very proud of the fact that she wore thermals because she didn't like overcoats. When she was going into the fitting room, she would say 'Just a minute, let me take my thermals off'. I think she enjoyed all that."
In his book 'The Glamour of Bellville Sassoon', the designer shows for the first time the Princess's thank you note for what she called her 'caring dress'. It also reveals her handwritten comments on Sassoon's sketches, charting her light-hearted relationship with the designer. Two of Sassoon's previously unpublished sketches from 1984 also show how, away from the public gaze, the Princess was already blossoming from a shy girl into the "red carpet glamour girl" remembered by posterity. The black jersey dress with a very low back and the pink blouson jacket with long skirt were, Sassoon said, a sign of things to come.
"They were for private use and a little more sexy, a little more fashion than what she could wear publicly at the time. Before her divorce, her clothes could never be overtly sexy. But just think how amazing she looked in the last years of her life. She was a red carpet glamour girl," he said.