Allied forces knew of the horrors of Nazi death camps more than two years earlier than is commonly thought, newly-released documents reveal.

The once-inaccessible archive of the United Nations War Crimes Commission (UNWCC), dating back to 1943, will be opened by the Wiener Library in central London. Members of the public will be able to read them online.

The documents show the UK, US and Soviet governments learned of the murder of millions of Jews as early as December 1942, but did little to provide sanctuary for those still at risk.

It had been thought the British government learned of the true extent of the Holocaust much later, when Allied forces discovered and liberated concentration camps near the end of Second World War.

Ben Barkow, the director of the Wiener Library, said: “This has never been a secret — no-one has covered this up. It’s just been neglected and overlooked.

“But this is important because it will change people’s understanding of this time, mostly on the level of detail it offers.”

In 1943 Viscount Cranborne, a minister in Winston Churchill’s war cabinet, said Jews should not be considered a special case, and that the British Empire was not able to admit any more.

Mr Barkow said it had been known that the British government was aware of the horrors of the Nazi regime because Anthony Eden, the then Foreign Secretary, made a speech to Parliament in December 1942 in which he said: “The German authorities, not content with denying to persons of Jewish race in all the territories over which their barbarous rule extends, the most elementary human rights, are now carrying into effect Hitler’s oft-repeated intention to exterminate the Jewish people.”

The files also show some of the first demands for justice came from Poland and China, rather than the US, Britain or the Soviet Union.

The release of the documents coincides with this week’s publication of Human Rights after Hitler: the Lost History of Prosecuting Axis War Crimes by Dan Plesch, the director of the Centre for International Studies and Diplomacy at Soas, University of London. Dr Plesch, who has been working with the documents for a decade under the close guard of the UN in New York, said another key finding was that Hitler was indicted as a war criminal by the UNWCC in 1944.

The commission had endorsed at least seven separate indictments against the Nazi leader by the time of his death, Dr Plesch said. The files also show the Polish government reported in detail to the UNWCC on the Auschwitz and Treblinka camps.

One of the UNWCC’s tasks was to collect evidence of war crimes for the arrest and trial of Axis war criminals. The commission was dissolved in 1949.

Mr Barkow said the documents were also suppressed in part because the Eastern Bloc countries, which were then Nazi-occupied, were “very active” in pursuing war crime charges — and this may have embarrassed Allied governments.

The Wiener Library was founded in Amsterdam in 1933 by Alfred Wiener, after he fled Germany. It transferred to central London in 1939.

The founding principle of the library was to document evidence of the Nazi persecution of the Jewish people.