William and Harry re-dedicated her grave in the grounds of the estate on what would have been her 56th birthday.
The Prince of Wales did not attend the ceremony – as he was busy on a Royal tour of Canada with his wife Camilla.
Glancing at Prince Charles on a visit to Prince Edward Island in Canada
“I still find it difficult now because at the time it was so raw.”
William to GQ
Reporters and photographers were not welcome at the deeply personal event which was held in total privacy.
Kensington Palace said in a brief statement: “The service, which will fall on what would have been the Princess’s birthday, will be conducted by The Archbishop of Canterbury and attended by her family.”
The two princes were joined by Wills’ beloved wife Kate and their two children Prince George and Princess Charlotte.
Diana died in a horrific car crash in Paris on August 31, 1997, when the pair were just 15 and 12.
Her marriage to Charles had been deeply troubled and rocked with scandal – eventually collapsing before their divorce in August, 1996.
Both Princes were both left traumatised after she died – with Harry slamming the decision to have him walk behind Diana’s coffin.
Princess Diana with Lady Sarah Armstrong-Jones
“I am in a better place about it than I have been for a long time, where I can talk about her more openly, talk about her more honestly, and I can remember her better, and publicly talk about her better.
“It has taken me almost 20 years to get to that stage. I still find it difficult now because at the time it was so raw.
“And also it is not like most people’s grief, because everyone else knows about it, everyone knows the story, everyone knows her.”
he wreckage of Princess Diana’s car in the Alma Tunnel
Both Harry and William have drawn on Diana’s tragic death as they campaign to raise awareness of mental health.
William is the second in line for the throne after Charles, while Harry is fifth after his father, brother and Wills’ two children.