The mother of Moors Murders victim Keith Bennett has died without knowing where her son is buried.
Winnie Johnson, 78, had been suffering from cancer for a number of years and passed away peacefully at a hospice earlier today, her lawyer said.
Her 12-year-son's killer, Ian Brady, has always refused to disclose the location of Keith's body on Saddleworth Moor above Manchester.
Keith was taken by Brady and his partner, Myra Hindley on June 16 1964, after he left home to visit his grandmother.
In a statement, John Ainley, a senior partner at Oldham-based solicitors North Ainley Halliwell, said: "Winnie Johnson, mother of the last missing Moors Murders victim Keith Bennett, has died without discovering her son's whereabouts.
"Winnie aged 78 passed away peacefully at a hospice after she had been diagnosed with cancer some years ago.
"It is a terrible tragedy and I would ask the media to respect Winnie's family's wishes to be left to grieve in private at this distressing time.
"She will be sadly missed and was much loved by her family and friends."
Mr Ainley had represented her legal interests for the last few years and has reaffirmed her beliefs that Brady still holds the key to the missing child's burial spot.
"Over the years and in all our personal meetings, Winnie has insisted Brady is the only person who could put her mind to rest and give her the chance to give Keith a decent burial before she passed away," he said.
"She was convinced Brady knew where her son was buried and she told me she wanted the search to continue to find Keith.
"Despite her personal appeals directly to Brady and via my office, Brady had persistently ignored the wishes of a grieving mother.
"She has died without knowing Keith's whereabouts and without the opportunity to finally put him at rest in a decent grave.
"It is a truly heartbreaking situation that this opportunity has now been irrevocably lost.
"Winnie's health deteriorated in the last few days. She died not knowing of the letter's possible existence but the steadfast conviction Ian Brady can resolve the situation."
Mr Ainley said she was "a most courageous lady".
He told Sky News: "She certainly was a most courageous lady, never gave up the hope that she would find Keith, and pursued that to the very end. She always thought that she would find Keith but without information from Ian Brady, that became more and more difficult."
Asked if her death would change anything, he said: "I think that any information that's passed over is going to be too little too late, sadly, certainly as far as Winnie's concerned, but having said that, if any information that would identify where Keith's buried, was passed to the police and the family, then certainly they would be able to give Keith a proper family burial, and that would give them some closure."
He said there were other close family members who would obviously continue fighting to find Keith.
On his website, http://www.searchingforkeith.com, Keith's brother, Alan Bennett, said on behalf of the family: "Winnie Johnson, the mother of Keith Bennett, died this morning at 12.35am, aged 78. She was a much loved mother, grandmother and great-grandmother, and is survived by one younger brother.
"Winnie fought tirelessly for decades to find Keith and give him a Christian burial. Although this was not possible during her lifetime, we, her family, intend to continue this fight now for her and for Keith. We hope that the authorities and the public will support us in this.
"We would like to express our deep gratitude to all the medical staff who cared for Winnie during her illness, and also the public for their good wishes and support over the years.
"We hope that everyone will understand, however, that we would like to grieve for her in private, both at this time and during her funeral."
Mrs Johnson's death comes amid a police investigation into whether a letter exists in which Brady apparently discloses the location of Keith's body.
Brady's mental health advocate Jackie Powell told a Channel 4 documentary the Moors Murderer gave her a sealed envelope to pass to Winnie Johnson in the event of his death.
Detectives examining a large number of documents seized from Ms Powell's home in South Wales have so far found no letter or any evidence to suggest Brady has disclosed the location of Keith's body.
A search of Brady's quarters at Ashworth Hospital in Merseyside has also failed to uncover the alleged letter.
It is understood Ms Powell claims she returned the envelope to Brady before her arrest on Thursday. She has since been released on bail pending further inquiries.
Martin Bottomley, head of investigative review of Greater Manchester Police's major and cold case crime unit, said: "Our thoughts are with Winnie's family at this very difficult and sad time.
"Winnie spent the majority of her life courageously fighting to get justice for Keith.
"All she wanted was to know where Keith was buried so she could lay him to rest.
"Sadly, almost all of her life, she has had to live with the knowledge that Ian Brady refused to show compassion and do the right thing and disclose where he had buried her little boy.
"Winnie was a tenacious and courageous woman and I wish that in her lifetime she could have found where Keith was buried.
"The only comfort we can take is that she is now at peace with the little boy she missed so much.
"We are continuing to examine the documentation we seized to establish whether or not this letter even exists.
"Notwithstanding that, there is only one person who knows where Keith is buried and that is Ian Brady himself. I would implore him at this extremely sad time and knowing that Winnie has died not knowing where Keith is buried to at last do the decent thing and tell us where he is."
Brady's lawyer, Richard Nicholas, declined to comment on the news of the death.
Brady, 74, and Hindley, who died in jail in 2002 aged 60, were responsible for the murders of five youngsters in the 1960s.
Their victims were sexually tortured before being buried on Saddleworth Moor.
Pauline Reade, 16, disappeared on her way to a disco on July 12 1963, and John Kilbride, 12, was snatched in November the same year.
Keith Bennett was snatched after he left home in Longsight on his way to his grandmother's house nearby. Lesley Ann Downey, 10, was lured away from a funfair on Boxing Day 1964, and Edward Evans, 17, was killed in October 1965.
Brady was jailed for life at Chester Assizes in 1966 for the murders of John, Lesley Ann and Edward.
Hindley was convicted of killing Lesley Ann and Edward and shielding Brady after John's murder, and jailed for life. In 1987 the pair finally admitted killing Keith and Pauline.
Both were taken back to Saddleworth Moor to help police find the remains of the missing victims but only Pauline's body was found.
Last month police were informed of the possible existence of a letter from Brady addressed to Winnie Johnson.
Ms Powell, 49, who is also an executor to Brady's will, told a Channel 4 documentary: "I received a letter and a sealed envelope which said on the front 'To be opened in the event of my death'.
"He says he doesn't wish to take his secrets to the grave and within the sealed envelope is a letter to Winnie Johnson.
"Within that is the means of her possibly being able to rest."
She was later questioned on suspicion of preventing the burial of a body without lawful exercise.
The programme, Ian Brady: Endgames Of A Psychopath, will be broadcast on Monday night at 9pm.
Brady was due to go before a mental health tribunal last month, but it was delayed after he suffered a seizure.
The tribunal was to consider his application to be transferred to a prison and be allowed to die. He has been tube-fed since refusing food 12 years ago.
No new date has been set for the hearing in Manchester but it is not thought to be imminent.