It was perhaps the bravest act of espionage of the Second World War. After voluntarily being imprisoned in the Auschwitz concentration camp for 2½ years, and smuggling out its darkest secrets to the Allies, Witold Pilecki overcame a guard and, with two comrades, escaped almost certain death.

Now new details have emerged of the extraordinary tale of the Polish officer who hatched a plot with the country's resistance to be rounded up by the occupying Germans in September 1940 and sent to the most notorious Nazi extermination centre.

At the time Auschwitz was predominantly a camp for captured resistance fighters, although Jews and anyone considered a threat to the Nazi regime were also being sent there.

Newly released documents from the Polish archives reveal how Mr Pilecki, going under the false name Tomasz Serafinski, went about setting up an underground resistance group in the camp, recruiting its members and organising it into a coherent movement.

“In order to assure greater security I have taken the view that each cell of five will not be aware of another cell,” he wrote in one of his reports smuggled out to the Resistance and which has now come to light.

“This is also why I have avoided people who are registered here under their real names. Some are involved in the most incompetent conspiracies and have their own plans for rebellion in the camp.”

Later he wrote: “The gigantic machinery of the camp spewing out dead bodies has claimed many of my friends ... We have sent messages to the outside world which were then transmitted back by foreign radio stations. Consequently the camp guards are very angry right now.”

Mr Pilecki's reports from the camp were channelled to the Allies via a courier system that the Polish Resistance operated throughout occupied Europe. By 1942 Mr Pilecki's organisation realised the existence of the gas chambers and he worked on several plans to liberate Auschwitz, including one in which the RAF would bomb the walls, or Free Polish paratroopers would fly in from Britain.

However, in 1943, realising that the Allies had no plans to liberate the camp, he and two others escaped. The new documents include a Gestapo manhunt alert after his escape.

Mr Pilecki ensured that a full report on the camp reached London, and the resistance group he started in Auschwitz continued to feed information to Britain and the United States, confirming that the Nazis were bent on the extermination of the Jews.

The archive material will again raise questions as to why the Allies, and in particular Winston Churchill, never did anything to stop the atrocities there. “We can only assume the British thought we were exaggerating,” said the Polish historian Jacek Pawlowicz. “I'm certain Poles shared their intelligence with MI6 and the highest levels of British Government, which, for some reason, remained silent.”

After his escape Mr Pilecki was captured fighting in the Warsaw Uprising in 1944 and spent the rest of the conflict in a prisoner-of-war camp. In July 1945 he joined Free Polish troops in Italy, from where he agreed to return to Poland and gather intelligence on the Soviet takeover of the country.

He was, however, caught by the Polish Communist regime. In a twist of fate, a Polish Jew administered the torture during his interrogation. Mr Pilecki's wife was invited to visit and he told her that his time in Auschwitz was child's play by comparison. After a show trial he was given three death sentences and shot.

The new material includes his charge sheet, which has 132 subsections, each listing a separate alleged crime. “From July 1945 to May 1947 the accused worked against the Polish state as a paid resident of an overseas intelligence agency,” one accusation reads. “The worst crime committed against the state was that he was acting in the interests of foreign imperialism, to which he has completely sold out through a prolonged period of work as a spy.” The implication is clear: Mr Pilecki was providing information on the Soviet-backed regime that was finding its way to MI6.

After his death Mr Pilecki was demonised by the Communists and his heroics re-emerged only after 1989.

His son, Andrzej Pilecki, who was 16 when he learnt that his father had been executed, said: “There'd be no better memorial to my father than for the young to learn of his example. I was at school at the time, it was a terrible shock, but now after 60 years of waiting I am thrilled to see justice.”

The new archive releases also reveal touching details. In a smuggled letter dated October 18, 1943, to his ten-year-old daughter he wrote: “I am very happy to hear you are such a devoted housemaid and that you like to take care of the animals and our plants in the garden. I, too, like every kind of bug and beetle as well as the beans and the peas. I like everything that lives. I'm very glad to hear that inside my children there are the same thoughts that I have.”

The Chief Rabbi of Poland, Michael Schudrich, said that Mr Pilecki was “an example of inexplicable goodness at a time of inexplicable evil. There is ever-growing awareness of Poles helping Jews in the Holocaust, and how they paid with their lives, like Pilecki. We must honour these examples and follow them today in the parts of the world where there are horrors again.”

The historian Michael R.D.Foot said that the life and death of Mr Pilecki brought shame on the British and the Allies, who turned a blind eye to Stalin's European ambitions as well as the Holocaust. “The Foreign Office's betrayal of Poland is the darkest chapter in its history, even if that betrayal was a strategic necessity,” he said.

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