Prince William speaks out about the loss of his mother Princess Diana

LONDON, April 19 (Reuters) - Britain's Prince William has said the shock of losing his mother Princess Diana two decades ago still lingers within him, days after his younger brother Harry spoke about his own struggles coming to terms with her death.

Diana was killed in a car crash in Paris in August 1997 when William was aged 15 and Harry was 12. The princes' comments this week have been their most intimate to date on the subject of their grief for their mother.

"The shock is the biggest thing and I still feel (it) 20 years later about my mother," William said in a BBC documentary to be aired on Thursday about mental health sufferers who will be running the London Marathon.

Princess Diana's passing:

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Princess Diana's funeral
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Princess Diana's funeral
Thousands of people queue up to sign the condolence book for Diana, Princess of Wales, at St James' Palace in London, September 1. The funeral of the Princess, who died in a car accident in Paris along with her friend Dodi Fayed and their driver, will take place on Saturday, Buckingham Palace announced today. BRITAIN DIANA
A Union flag flies at half mast infront of Westminster Abbey, September 1, where the funeral service for Diana, Princess of Wales will be held next Saturday. Diana was killed in a car accident in Paris, August 31, with her friend Dodi Al Fayed and their driver. BRITAIN DIANA
Tourists pass postcards of Princess Diana and Prince Charles at a Marylebone street souvenir shop in London, September 1. The funeral of the princess, who died in a car accident in Paris along with her friend Dodi Fayed and his driver, will take place on September 6. DIANA
A large crowd gathers in front of the Buckingham Palace in London, September 2, to pay tribute to Princess Diana. [The funeral for the Princess, who died in a car accident in Paris on Sunday along with her friend Dodi Al Fayed and his driver, will take place on September 6].
Mourners surround a carpet of flowers outside Kensington Palace September 4 where thousands of mourners have been gathering all week to lay flowers and pay their last respect to Diana, Princess of Wales. The funeral for the Princess, who died in a car accident in Paris along with her friend Dodi Al Fayed and his driver, will take place on September 6. DIANA
Charles, Prince of Wales accompanied by his two sons, Prince Harry (L) and Prince William, thank well-wishers for their tributes to Diana, Princess of Wales, outside Kensington Palace September 5. Spontaneous applause broke out as the princes appeared through a side gate and began to inspect the sea of flowers on the eve of their mother's funeral.
The hearse bearing the coffin of Diana, Princess of Wales makes its way through the crowd gathered outside Buckingham Palace after the funeral service at Westminster Abbey, September 6. A million people lined the funeral route through London to pay their last respects to the Princess. DIANA
Prince Charles (R) puts his hand on Prince Harry's (C) shoulder as Prince William looks on after the coffin of Diana, Princess of Wales, was placed into a hearse September 6. Millions of mourners lined the streets of central London to watch the funeral procession. The Princess died last week in a car crash in Paris. DIANA
Crowds gather to watch on a giant video screen in Hyde Park the funeral service of Diana, Princess of Wales, at Westminster Abbey September 6. Millions of people lined the streets of London to watch the funeral procesion of the Princess who died in a car crash last week in Paris. DIANA
Earl Spencer (2L) Prince William (3L) , Prince Harry and Prince Charles (R) watch as the coffin of Diana, Princess of Wales is placed into a hearse at Westminster Abbey following her funeral service, September 6. Millions of mourners lined the route to pay their respects. DIANA
The Queen (2ndR) and the Queen Mother (L) wait with members of the clergy after the funeral of Diana, Princess of Wales, September 6. Millions of mourners lined the route taken by the funeral procession. DIANA
Prince Harry, Prince Charles, Earl Spencer and Prince William (L to R) leave Westminster Abbey after a funeral ceremony for Diana, Princess of Wales, September 6. Hundreds of thousands of mourners lined the streets of central London to watch the funeral procession. The Princess died last week in a car crash in Paris. DIANA
The hearse carrying the coffin of Diana, Princess of Wales, makes its way to a private burial at Althorp following the service at Westminster Abbey, September 6. Millions of people lined the streets of London to watch the funeral procession of the Princess who died in a car crash last week in Paris. DIANA
Mourners weep as the funeral cortege of Diana, Princess of Wales, passes on its way to Westminster Abbey September 6. Millions of people lined the streets of London to watch the funeral procession of the Princess who died in a car crash last week in Paris. DIANA
Mourners react as the gun carriage bearing the body of Diana, Princess of Wales, passes by September 6. Millions of mourners lined the streets of central London to watch the funeral procession. The Princess died last week in a car crash in Paris. DIANA
The hearse carrying the body of Diana, Princess of Wales, travels back down The Mall following the funeral service at Westminster Abbey September 6. Millions of people lined the streets of London to watch the funeral procession of the Princess who died in a car crash last week in Paris. DIANA
Pop-star Elton John sings a rewritten version of his song 'Candle in the wind' as a tribute to Diana, Princess of Wales at her funeral, September 6. Over a million mourners lined the route of the funeral procession through London. DIANA
The coffin bearing Diana, Princess of Wales, is carried by Welsh Guardsmen through Westminster Abbey during her funeral service, September 6. Over a million people lined the route of the funeral procession through London. DIANA
File photo dated 06/09/97 of Elton John (right) and George Michael (left) leaving Westminster Abbey in London, after the funeral service for Diana, Princess of Wales. Pop superstar Mr Michael has died peacefully at home, his publicist said.
The Duke of Edinburgh, Prince William, Earl Spencer, Prince Harry and the Prince of Wales follow the coffin of Diana, Princess of Wales in September 1997.
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"I still have shock within me. People go 'shock can't last that long' but it does. You never get over it, it's such an unbelievably big moment in your life that it never leaves you. You just learn to deal with it," William said in excerpts released on Wednesday.

Both royal brothers have spoken out as part of their campaign to raise awareness of mental health challenges and to encourage people to speak openly rather than bottle up their emotions.

On Tuesday, William released a video chat with U.S. pop star Lady Gaga, who has battled post-traumatic stress disorder (PTSD) after she was raped aged 19, and warned British men that keeping a "stiff upper lip" and not speaking about their feelings was detrimental to mental health.

Harry said in a newspaper interview on Monday that he had come close to a "complete breakdown" on several occasions and had sought counseling in his late twenties to help deal with the grief of losing his mother. (Reporting by Michael Holden, editing by Estelle Shirbon)

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