(01 of10)
Open Image ModalOne of the earliest sources of information was British intelligence from the codebreakers at Bletchley Park in Buckinghamshire, who worked in totally secrecy to crack encrypted enemy messages.
German police radio messages first revealed atrocities taking place on the Eastern Front in 1941, though only the most senior figures such as Alan Turing would have known all of the information discovered by the messages.
(02 of10)
Open Image ModalBritain was certainly aware of the Nazi persecution of the Jews - though not necessarily the extent of their plans - before the Second World War even started.
News reached the UK of the violent pogrom against Jews in Germany on 9 November 1938, known as Kristallnacht (Night of Broken Glass). The Nazi authorities organised the burning of synagogues, looting of Jewish businesses, schools and hospitals and the murder of dozens of Jewish people.
The British government took action, easing immigration restrictions for certain categories of refugees. A rescue programme called Kindertransport (Children's Transport) brought thousands of refugee children to Britain, from German, Poland, Austria and Czechoslovakia.
Between 9,000 and 10,000 children were transported, around 7,500 of them were Jews. (credit:ASSOCIATED PRESS)
(03 of10)
Open Image ModalIn 1942, reports of a Nazi plan to murder all Jews reached Allied leaders from several sources. The harrowing news included details on methods, numbers, and locations for the plans.
One report came from The General Jewish Labour Bund, a socialist group with a base in the Warsaw ghetto. (credit:ASSOCIATED PRESS)
(04 of10)
Open Image ModalIn summer 1942, Gerhart Riegner, the secretary-general of the World Jewish Congress based in Switzerland, sent a cable to Stephen Samuel Wise, the congress president. It was sent on diplomatic channels and reported Nazi plans for a Holocaust.
His information came from Eduard Schulte, the man who owned a German company that employed high-level Nazi officials. The telegram is regarded as the first official communication about the plans.
It was passed on to the British Foreign Office and the US state department. (credit:Armchair Aviator/Flickr)
(05 of10)
Open Image ModalBritain's wartime leader Winston Churchill was aware of the Holocaust. He was a vocal supporter of The Jews, according to Martin Gilbert, CBE, a Jewish historian and Churchill's biographer. Gilbert claims Churchill took steps to place pressure on Germany and help those in danger, and even pushed to bomb Auschwitz but was overruled.
Though some Jewish groups say he didn't go far enough, he supported moves like a boycott of German goods before he became prime minister, and opposed the prevention of Jewish refugees reaching Palestine when he was in office. (credit:Peter Macdiarmid via Getty Images)
(06 of10)
Open Image ModalMore information was also collected in the autumn of 1942, from eyewitnesses. This included first hand testimony from a Polish underground courier and accounts from 69 Polish Jews who arrived in Palestine, in a prisoner exchange between Germany and Britain. (credit:ASSOCIATED PRESS)
(07 of10)
Open Image ModalOn 17 December 1942, Britain and the Allies issued a proclamation condemning the "extermination" of the Jewish people.
The declaration warned Nazi Germany would be held responsible for the crimes - although historians say it is still not clear how much the Allies understood the scale of the killings, and the importance of the information they had. (credit:ASSOCIATED PRESS)
(08 of10)
Open Image ModalOn 18 April 1945, US army chief General Eisenhower telephoned Churchill to say that American troops had entered concentration camps. Churchill told the House of Commons that his government felt "horror" at the "proofs of these frightful crimes now coming into view."
When Allied forces entered the camps, their horror at the bodies and the conditions of the few survivors has been taken as a sign that they did not understand the full extent of the atrocities, despite the information that they had, according to the Jewish Virtual Library. (credit:ASSOCIATED PRESS)
(09 of10)
Open Image ModalBut whatever the Allies knew, US Holocaust historian David Wyman has questioned whether they could have done more to stop the mass murder.
Leaders have often been accused of failing to respond quickly enough to the news. Some historians such as Wyman argue that moves like bombing the gas chambers of Auschwitz would have lessened the number of deaths, but others say it would have had no effect on the Nazi genocide.
Large-scale rescue operations were considered but it was not clear to policymakers how this could be achieved. (credit:Ian Forsyth via Getty Images)
(10 of10)
Open Image ModalBritish media such as the BBC did report on the Holocaust, more visibly than in some other countries. The New York Times was criticised by journalist Laurel Leff for its lack over coverage which "contributed to the public's ignorance".
After Auschwitz and other camps were liberated in 1945, Britain's media outlets gave the concentration camps major prominence in news reports.
A BBC directive from 20 April 1945 - two days after Churchill was told troops had entered the camps - lists "concentration camps" as the top item in a morning news conference.
It notes an article from The Times which contained photographs "to confirm the written descriptions" of Nazi brutality. The article emphasised that it was essential for British people to fully comprehend the horror of the camps through descriptions and photographs. (credit:George Konig via Getty Images)