The Duke of Cambridge has revealed the shock of his mother Diana's death lives with him to this day.

Prince William made the admission in the Mind Over Marathon documentary, which charts the efforts of a group of runners with mental health problems as they train for the London Marathon.

In the film William, along with his wife Kate and hir brother Prince Harry, attend one of the runners' training sessions.

There the prince sympathised with a London Marathon runner, Rhian Burke, 39, who asks for advice about comorting her own bereaved children.

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."

Ms Burke suffered the loss of her one-year-old son George, who died in 2012 after contracting pneumonia, followed within five days by the loss of her husband, Paul, who took his own life.

In the film the prince 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 on Tuesday 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 Diana Princess of Wales.

The Marathon takes place this Sunday.