The new memorial, which is located in the Sunken Garden at the London palace, was revealed in a private ceremony on Thursday 1 July.
The date marks what would have been the 60th birthday of the Princess of Wales, who died at the age of 36 in August 1997 in Paris.
After a much-reported rift between the royal brothers, Prince William and Prince Harry were reunited for the occasion to pay joint tribute to their mother.
They were photographed standing next to one another, and laughing with each other, at the outdoor ceremony.
They were joined by members of the Spencer family, including Diana’s brother, Earl Spencer, and Diana’s two sisters, Lady Sarah McCorquodale and Lady Jane Fellowes.
The statue was created by sculptor Ian Rank-Broadley, who has an established relationship with the royal family.
The Sunken Garden - said to be a favourite place of the Princess during her time living at the palace - was also redesigned for the date, with the inclusion of 4,000 new flowers.
The statue was first commissioned by the brothers several years ago, in 2017, to mark 20 years since Diana’s death.
In a joint statement the brothers said: “Today, on what would have been our mother’s 60th birthday, we remember her love, strength and character - qualities that made her a force for good around the world, changing countless lives for the better.
“Every day, we wish she were still with us, and our hope is that this statue will be seen forever as a symbol of her life and her legacy.
“Thank you to Ian Rank-Broadley, Pip Morrison and their teams for their outstanding work, to the friends and donors who helped make this happen, and to all those around the world who keep our mother’s memory alive.”
The bronze statue of Diana depicts the princess with three children to represent the “universality and generational impact” of her work, said a statement.
In front of the statue is a paving stone engraved with an extract inspired by The Measure of A Man poem. It reads: “These are the units to measure the worth.
“Of this woman as a woman regardless of birth. Not what was her station? But had she a heart? How did she play her God-given part?”
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