The mother of Moors Murders victim Keith Bennett has died without knowing where her son's remains were left after his death in the 1960s.
Winnie Johnson, 78, who had been suffering from cancer, died peacefully at a hospice on Saturday, her lawyer said.
Ian Brady, the killer of her 12-year-son, had 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, lawyer John Ainley, 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 Johnson's legal interests in recent years and has reaffirmed her beliefs that Brady still holds the key to the missing child's burial spot.
There have been reports in recent days that Brady may have revealed the location of Keith's grave in documents passed to his legal advocate, with strict orders the envelope was to be opened only after his death.
"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," Ainley 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."