A group of fans gathered outside the gates from 5:30 a.m. to hold a vigil for the late princess, who was tragically killed in a car crash in 1997.
The 37-year-old prince astounded the well-wishers when he emerged through the palace gates to speak to them.
The small group who had brought flowers and bunting were left emotional when William shook their hands and thanked them for their support.
One fan, John Loughrey, 59, told the Daily Mail: “William told me he knew we'd been coming here for years and thanked us for what we were doing for his mother.”
“I'm still shaking now. I feel very emotional,” he added.
Loughrey said William admired his collection of badges pinned to his hat and said he thought it was “wonderful” that they were celebrating his mother's life and legacy.
“He said, 'I'm touched by what you do, you've got quite the collection.’ I told him that I pray for his mother every Sunday at Westminster Abbey and he seemed really touched.”
William and brother Prince Harry were just 15 and 12 when their mother died.
A statue of Diana will be erected in the grounds of Kensington Palace at the request of her sons. The brothers commissioned the project in 2017 to mark the 20th anniversary of her death, but its completion has been delayed, as they want to make sure the monument is “completely right.”
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