The Glasgow-born killer said in a letter from his cell in a high-security hospital that too much time had passed for him to be able to pinpoint the 12-year-old's final resting place.
Brady, along with his accomplice and girlfriend Myra Hindley, tortured and murdered Bennett in 1966 before burying his remains on Saddleworth Moor outside Manchester.
The boy is the only one of Brady and Hindley's five victims whose body has not been recovered.
This summer the killer's mental health advocate claimed Brady had given her a letter saying where Bennett could be found. She said she returned the letter unopened, but her claim sparked a Channel 4 documentary that concluded Brady was trying to manipulate people from behind bars.
However, in another letter to Scottish journalist Jean Rafferty, Brady denies ever sending such a letter.
He wrote: "Since 1985 I have repeatedly stated that I know/knew the whereabouts of Keith Bennett. But to know or have known the whereabouts is entirely distinct from stating that a vast wilderness that changes with the seasons every year would, after 50 years, remain precisely familiar to anyone."
Brady has not seen Saddleworth since 1987, when he was let out to try to show police where to find Bennett. The boy's mother, Winnie Johnson, has since died without learning where her child lies.
Brady's handwritten letter shows contempt for the makers of the Channel 4 documentary.
He refers to "the yellow dog-eared psycho-babble of discredited penal quacks, seeking to build a media career from self-publicity and exploitation of ancient criminal relics ... (It) cynically added to the grief of relatives by deliberate blackmail of witnesses and obfuscation of common facts, long documented, by causing flustered misinformation, re a sealed letter that never existed, to further muddy the waters".
He added: "Knowing the where and when of the past is clearly quite different from stating the where and when of the present.
"As any hack reporter would axiomatically know, and only a deliberately fraudulent one would try to obsfuscate and exploit for personal gain ... Given the common knowledge of ever-changing heath and bog, what calibre of 'reporter' would believe in the existence of a letter capable of identifying a spot in a yearly changing wilderness"
Neither Jackie Powell – Brady's mental health advocate – nor Channel 4 were available for comment.
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