The Duke of Cambridge has revealed the shock of his mother's death lives with him to this day as he sympathised with a London Marathon runner who appealed for advice on how to comfort her bereaved children.
William told Rhian Burke, 39, who suffered the double loss of her one-year-old son and husband within days of each other, that her son and daughter would be well looked after as she would "provide the blanket of stability and understanding that they need."
Speaking about the death of his mother Diana, Princess of Wales, he told her: "I still feel, you know, 20 years later about my mother, I still have shock within me - 20 years later."
The Duke chatted to the mother during the BBC1 documentary Mind Over Marathon, being screened on Thursday evening, that charts the efforts of a group of runners with mental health problems who are training for the London Marathon, which takes place on Sunday.
The group are running for William, Kate and Harry's Heads Together mental health campaign, which is the London Marathon's charity and aims to encourage people to speak out about their psychological problems or be sympathetic to those in need.
At a preview screening of the documentary on Tuesday, the Duke caught up with the runners and in an unscripted speech told them the death of his mother spurred him on tackle the issue of mental health.
He said: "As you can see, you know, I have my own reasons for being involved in mental health, what happened to me with my mother when I was younger, but equally the charitable work I do at the moment and the areas that I'm involved in, it all comes back to mental health.''
In the documentary, the first instalment of a two-part programme, presenter Nick Knowles follows the progress of the 10 runners whose psychological problems range from post traumatic stress disorder to anxiety and depression, but who hope running can help their conditions.
William, Kate and Harry join the group for a training session a number of weeks into their challenge and the Duke speaks to Ms Burke, whose toddler George died after contracting pneumonia in 2012 before, five days later, her husband Paul took his own life.
When she asked if her young son and daughter would be fine, despite their loss, the Duke replied that because she was aware of the issues that could affect her children, she was "already a step ahead of what could happen".
The 20th anniversary of Diana's death is this summer. William was 15 years old and Harry just 12 when she was killed in a car crash in Paris on August 31 1997.
Speaking about his mother, the Duke said: " People go 'shock can't last that long' but it does, you never get over it. Such an unbelievably big moment in your life, that it never leaves you, you just learn to deal with it."
In the programme, the runners are seen progressing with their training with Ms Burke overcoming her anxiety enough to run solo while others reach other fitness goals, with only one dropping out for a number of reasons including injury.