To mark the 21st anniversary of Princess Diana's death, her close friend Rosa Monckton has shared a stunning never-before-seen picture of the late royal. The image, taken prior to her divorce from Prince Charles, was posted to Rosa’s Twitter account and shows an off-duty Diana looking happy and carefree. "Diana as I remember her. Not enough is written about her sense of fun and zest for life. Friend and wonderful godmother to Domenica. RIP," wrote Rosa, who was with her friend just two weeks before her untimely passing on Aug 31 1997.

The heart-warming snap is a reminder of Diana's youthful spirit and fun-loving attitude. Dressed down in jeans and a baggy sweater, a far cry from her inimitable black tie ensembles, the People's Princess is seen laughing with her eyes closed while lounging on what looks to be a private plane.

Diana as I remember her. Not enough is written about her sense of fun and zest for life. Friend and wonderful godmother to Domenica. RIP pic.twitter.com/WP49dNDeEu

— Rosa Monckton (@MoncktonR) August 31, 2018

Rosa and Diana met through mutual friends and shared an extremely close bond. Their friendship extended to their families, with Princes William and Harry's mother acting as godmother to Rosa's daughter Domenica, who was born with Down syndrome. Speaking to People magazine in 2016, the British jewelry executive reflected on the loving words of wisdom she received from her friend following her daughter's birth, sharing, “She said, ‘Believe in her, love her and I’ll be there every step of the way.’ She already had that vision of what Domenica could become, which I simply couldn’t see."

Related: How Princes William and Harry are keeping their mother's legacy alive

Princess Diana passed away on August 31 1997.

With her daughter acting as inspiration, Rosa formed the charity Team Domenica to help young adults with learning disabilities find employment. Now in her 20s, Diana's goddaughter is thriving and plays an active role in the organization, something Rosa says her friend would be delighted to see – “[Diana] would be so proud of Domenica and what she’s achieved and of this."