Prince Harry is to make his first public appearance since admitting he sought counselling to come to terms with the death of his mother, Diana, Princess of Wales.
Harry will officially open this year's London Marathon Expo and meet runners and hand out race numbers to those taking part in the gruelling 26-mile challenge on Sunday.
The Prince received widespread praise for admitting, in an interview with the Daily Telegraph, he spent nearly 20 years "not thinking" about his mother's death and eventually got help after two years of "total chaos''.
Prime Minister Theresa May said his decision to open up would help "smash the stigma around mental health,'' Mind, the mental health charity, described it as a "true turning point,'' while Sir Elton John tweeted it was a "sign of strength to ask for help".
At the Marathon Expo, being held at the ExCeL centre in London's Docklands, Harry, who is Patron of the London Marathon Charitable Trust, will also hand out special edition Heads Together headbands.
Heads Together is the mental health campaign spearheaded by Harry and the Duke and Duchess of Cambridge with the aim of encouraging the nation to talk about their psychological problems or be a sympathetic ear for those in need.
The royal trio are campaigning hard on the issue this week in the run up to the London Marathon which has made their campaign it's official charity.
Every runner will receive a blue Heads Together headband, along with a note from William, Kate and Harry wishing them luck and encouraging them, no matter which important cause they are running for, to help Heads Together make the event the mental health marathon.
Later Kate will host marathon runners from team Heads Together at Kensington Palace, where she will chat to the group and discuss their reasons for taking part and wish them luck.
The runners will then take part in a training session with coach Nick Anderson.