The mother of Moors Murders' victim Keith Bennett, Winnie Johnson, has died aged 78.
Ms Johnson 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-old son's killer, Ian Brady, has always refused to disclose the location of his body on Saddleworth Moor above Manchester.
Keith was taken by Brady and his partner, Myra Hindley, on 16 June 1964 after he left home to visit his grandmother.
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 that 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.
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.
"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."