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 from Neyland 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 George and husband Paul 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 Rhian: "I still feel, you know, 20 years later about my mother, I still have shock within me - 20 years later."

The Duke chatted to Rhian 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.

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 Rhian, 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 Rhian 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.

Following George's death, Rhian set up the charity 2 Wish Upon A Star to improve bereavement services in Wales.

The charity has been a huge success and Rhian has won numerous awards for her charity work, including a prestigious St David Award.

But Rhian said behind the scenes she has been facing a real battle with Post Traumatic Stress Disorder and anxiety.

Rhian said: "Each of us have been battling mental illness and decided to get involved to help raise awareness of something that is still rarely spoken about.

"I decided to get involved in this documentary to highlight the effects that sudden loss and bereavement can have on the mind and to be able to discuss the day to day battles I have faced since losing my boys five years ago. PTSD and anxiety have crippled me and I felt I wanted to let others know how this makes you feel but also that you can beat it."

Rhian appeared on BBC Breakfast this morning (Wednesday) and will feature on Breakfast with Lorraine on ITV tomorrow (Thursday) morning.

Rhian added: "It would mean so much to me if you could tune in, share and support my new friends and I on the journey that we have experienced the last six months. It has helped me more than words, given me the confidence to continue fighting on and has given me hope that I can beat this..

"I can do this - run 26.2 miles whilst helping so many others hiding behind the mask I have worn for so long."

  • The documentary Mind Over Marathon is being aired on BBC One on Thursday April 20 at 9pm and the second part on April 27 at 9pm.
  • You can sponsor Rhian by clicking here.