This is not true. Victor Gollancz wrote a pamphlet called Let My People Go, published in January 1943, which included statements such as: "The murders have taken the form of random shooting, mass shootings, mass electrocutions, mass poison-gassing, and transportation in conditions which inevitably involve death during the journey" and "All this is part, not of war, but of a quite deliberate policy, openly proclaimed, of extermination of the Jewish population of Europe".
The pamphlet was widely circulated, and over 100,000 copies were sold, so the statements it contained were not secret. Whether people believed what was published, or whether they then did the right things, are matter for debate, but there cannot be any doubt that they had been told what was happening.
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