The Duke of Cambridge has said the shock of his mother Princess Diana's death still lives with him to this day.
It comes just days after his brother Prince Harry revealed he nearly suffered a mental breakdown in his Twenties as he came to terms with losing his mum.
Diana died in a car crash in Paris in 1997 when William and Harry were just 15 and 12.
William made the revelation while speaking to a London marathon runner who asked for advice comforting her own bereaved children.
He told Rhian Burke: "I still feel, you know, 20 years later about my mother. I still have shock within me - 20 years later."
Ms Burke lost her one-year-old son George, who died in 2012 after contracting pneumonia.
This was followed by the loss of her husband, Paul, who tragically took his own life five days later.
Her story was revealed in a film for the documentary Mind Over Marathon, which charts the efforts of a group of runners with mental health problems as they train for the London Marathon.
In the film, Prince William told Ms Burke that her son and daughter would be well looked after as she will "provide the blanket of stability and understanding that they need".
He said that because Ms Burke knew the issues that could affect her children, she was "already a step ahead of what could happen".
The group are running the Marathon for William, Kate and Harry's Heads Together mental health campaign, which aims to encourage people to speak out about their psychological problems or be sympathetic to those in need.
The BBC1 film airs on Thursday, but at a preview screening William, in an unscripted speech, spoke of how his mother's death had spurred him on to tackle the issue of mental health.
This summer marks the 20th anniversary of the death of Princess Diana.
The London Marathon takes place this Sunday April 23.