A secret intelligence report compiled just as Hitler embarked on the Final Solution concluded that the Nazi leader had a "messiah complex".
The document, drawn up for British intelligence in April 1942 just as the conflict was starting to turn against the dictator, has lain unread since the war.
It reveals how British analysts had noticed developing paranoia in his speechmaking and a growing preoccupation with what he called "the Jewish poison".
Just weeks after the analysis was compiled, senior Nazis set in place plans for the Final Solution - an intensification of the mass extermination of Jews.
Experts say the papers show British secret services sensed that, as the war turned against him, Hitler would resort to increasingly drastic measures.
The document was found among a collection of papers belonging to the family of Mark Abrams, a social scientist who worked with the BBC's overseas propaganda
analysis unit and the psychological warfare board during the war.
Written by Joseph MacCurdy, a Cambridge academic, it refers to earlier signs of "morbid tendencies", classifying these as "Shamanism", "epilepsy" and "paranoia".
The first referred to Hitler's hysteria and compulsion to feed off the energy of Nuremberg Rally-style audiences.
By 1942, Mr MacCurdy said this was in decline, and his report refers to the "dull
flatness of the delivery" but the report said the other two tendencies were developing.
"Epilepsy" referred to Hitler's cold and ruthless streak as well as a tendency to lose heart when his ambitions failed.
The analyst felt the failure of Operation Barbarossa - the name given to Hitler's invasion of Russia - had exposed this fatalism and wrote that Hitler's speech betrayed "a man who is seriously contemplating the possibility of utter defeat".
Hitler 's growing paranoia - his belief that he was leading a chosen people on a crusade against an evil incarnate in the Jews - was the most alarming, Mr MacCurdy wrote.
He said: "Hitler is caught up in a web of religious delusions. The Jews are the incarnation of evil, while he is the incarnation of the spirit of good.
"He is a god by whose sacrifice victory over evil may be achieved. He does not say this in so many words, but such a system of ideas would rationalise what he does say that is otherwise obscure."
The paper came to light after Dr Scott Anthony, who is working on the history of public relations at the University of Cambridge, began tracking down Dr Abrams' peers and relatives.
"At the time that it was written, the tide was starting to turn against Germany," Dr Anthony said. "In response, Hitler began to turn his attentions to the German home front.
"This document shows that British intelligence sensed this happening. MacCurdy recognised that, faced with external failure, the Nazi leader was focusing on a perceived 'enemy within' instead - namely the Jews.
"Given that we now know that the Final Solution was commencing, this makes for poignant reading."
An archive of documents about Dr Abrams' life and work is held by the Churchill Archives, University of Cambridge.
His family have added the original copy of the psychoanalysis to this collection, making it available to researchers for the first time.