Agents planned to smuggle doses of oestrogen into his food to make him less aggressive and more like his docile younger sister Paula, who worked as a secretary.

He explained that oestrogen was chosen because it was tasteless and would have a slow and subtle effect, meaning it would pass Hitler’s food testers unnoticed.

The Allied plot to turn Herr Hitler into Her Hitler was just one of a number of wacky ideas cooked up to break the stalemate, according to a new book.