The new evidence suggesting that Britain was aware of Witold Pilecki's plans to liberate Auschwitz will reignite the long-running debate over how much Winston Churchill knew about the death camp and whether he did enough to prevent the genocide taking place there.

There is little doubt that Churchill, in contrast to many of his contemporaries, was a staunch defender of the Jews and one of the few statesmen to grasp the enormity of the Holocaust.

As early as 1941 the code-breakers at Bletchley Park had furnished Churchill with ample evidence of the systematic mass murder of Jews. By 1942 he was condemning what he called “a bestial policy of cold-blooded extermination”. More specifically, he knew that a train containing 4,000 Jewish children had left Lyon for “somewhere in Poland”.

“There is no doubt,” he wrote to Anthony Eden, “that this is probably the greatest and most horrible crime ever committed in the whole history of the world, and has been done by scientific machinery by nominally civilised men in the name of a great State and one of the leading races in Europe.”

Sir Martin Gilbert, Churchill's official biographer, argues that it was not until July 1944 that Churchill learnt of Auschwitz, when he was also informed that Hungarian Jews were being transported there at the rate of 12,000 a day.

Responding to a plan to bomb Auschwitz from the air, he told Eden: “Get anything out of the Air Force you can, and invoke me if necessary.” The camp was within range of US bombers and several nearby military targets were destroyed from the air. Yet the rail lines to Auschwitz were never bombed. Churchill's defenders insist that his orders became bogged down in the Whitehall machinery, which was desperately focused on winning the war by military means. The decision not to bomb was apparently taken for “operational reasons” that have never been fully explained.

Churchill would claim that the full extent of the horror was not appreciated until much later: “I had no idea, when the war came to an end, of the horrible massacres which had occurred; the millions and millions that have been slaughtered. That dawned on us gradually after the struggle was over.”

Churchill's detractors insist that, for all his vocal support of the Jews, his practical assistance was strictly limited. Chaim Weizmann, the Zionist leader, claimed that Churchill and other Allied leaders had ignored his pleas for intervention to stop the killing.

“Nobody cared what happened to the Jews,” he said. “Nobody had raised a finger to stop them being slaughtered.”

At the Holocaust Memorial in Jerusalem last year, the former US President George Bush was blunt about the Allies' failure to destroy, or even interrupt, the Auschwitz death machine: “We should have bombed it.”

Churchill was also in no doubt as to what should happen to the operators of the death camps when the war was over. On July 11, 1944, he wrote: “All concerned in this crime who may fall into our hands, including the people who only obeyed orders by carrying out these butcheries, should be put to death.”

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