Led by the Archbishop of Canterbury, Justin Welby, the service comes as the princes move ahead with plans for a statue of their mother, which was commissioned earlier this year and will be erected at Kensington Palace.
Despite Althorp Estate's defence of these conditions in a statement, there began a multimillion-dollar project to redevelop the island on which Diana is buried.
This year marks 20 years since the tragic passing of Princess Diana.
And in a touching tribute Kensington Palace have confirmed Prince William and Prince Harry will head to their mother's resting place at her family home, Althorp House in Northamptonshire, to re-dedicate her grave.
The 31-year-old was 12 when Diana died in a vehicle crash in Paris in 1997.
Diana's ex-husband Prince Charles will not be present, due to a scheduled royal visit to Canada with Camilla, the Duchess of Cambridge.
After her funeral at Westminster Abbey in London 20 years ago, Diana's casket was driven to Althorp for its burial.
'My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television'. "I don't think it would happen today".
Both princes have begun talking recently, for the first time in public, about the trauma inflicted by their mother's death.
Spencer recalled how "furious" Diana was one time when she had taken Harry to school and a reporter who was filming Harry represented him as a rude child in a story after Harry stuck his tongue out at the camera. The award was given to young people, recognizing them for being a trailblazer in helping others and making the world a better place.
Do you remember where you were when you heard that Princess Diana died?