Just after Witold Pilecki’s arrival at Auschwitz concentration camp in September 1940, Deputy Commandant SS-Hauptsturmführer Karl Fritzsch addressed the 5,000 prisoners: “Your Poland is dead forever and now you are going to pay for your crimes through work,” he declared. “Look there, at the chimney. Look!” he shouted. “This is the crematory. Three thousand degrees of heat. The chimney is your only way to freedom.” In case anyone had not got the message, guards then beat a man senseless with clubs in front of them all.

A former cavalry officer in the Polish army, Pilecki was a member of the resistance in Warsaw. When news reached them in July 1940 that a concentration camp had been opened in a former Polish army barracks near the town of Oświęcim, Pilecki volunteered to be captured and taken to the camp – which the Germans called Auschwitz – in order to gain intelligence about what was happening there and to organise a break out. Few men had returned alive from Auschwitz and it was an incredibly brave decision. When a prisoner learned that Pilecki had volunteered to be there he was astonished: “If what you say is true you’re either the greatest hero or the biggest fool.”

Pilecki quickly began organising an underground network in Auschwitz, but conditions were far worse than he had imagined. There were constant beatings, humiliations and a starvation diet, all designed to break down bonds between prisoners and to destroy them psychologically as well as physically. According to Pilecki, “some slithered into a moral swamp. Others chiselled themselves a character of finest crystal.” He was certainly in the latter group.

By October prisoners were dying at the rate of about a dozen a day. Pilecki managed to smuggle a message out to the resistance in Warsaw about the “monstrous torture” being endured by prisoners. He was confident they would act. On Christmas Eve the guards put up a tree in the camp festooned with lights: “For a joke, the SS had stacked as presents under the tree the bodies of prisoners who had died that day in the penal company, mostly Jews.”

Witold Pilecki photographed in Auschwitz. Photograph: Courtesy of PMA-B.

Despite Pilecki’s optimism, as the months turned into years it became clear that help was not coming. Yet he did not give up. Each time someone escaped or was released, they carried reports for the Warsaw resistance. In October 1941 he detailed the gassing of Soviet prisoners using Zyklon B: “He had witnessed the creation of the camp’s first gas chamber with the power to kill on an industrial scale.”

Next he reported on the creation of a new camp nearby, called Birkenau in German after the silver birch copses there. From the spring of 1942, trainloads of Jewish men, women and children arrived at Auschwitz to be gassed; 35,000 would be killed that summer alone. Later he would write: “I would say that we have become animals … but no, we are a whole level of hell worse than animals.”

The resistance in Warsaw passed on his reports to the Polish government in exile, and they begged the allies to act. The evidence that Auschwitz had become the “epicentre of a vast, mechanised genocide unparalleled in human history” was there. But neither Roosevelt nor Churchill “considered that the murder of Jews demanded a direct response”. Once, after the Polish government in exile had again asked the allies to consider bombing Auschwitz, a British diplomat said tetchily: “The Poles are being very irritating over this.”

Journalist Jack Fairweather’s account of Pilecki’s bravery, endurance and humanity is the well deserved winner of the 2019 Costa biography award. As he says, Pilecki’s story is “essential for our understanding of how Auschwitz came into being”. The concentration camp was transformed into a death factory before Pilecki’s eyes. During nearly three years in Auschwitz, he risked his life to reveal the camp’s true horror. His heroic attempts to alert the world to what was happening were only fully revealed after the fall of the Soviet Union and the opening of the Polish state archives.

He appeared in Poland’s first Soviet-style show trials in March 1948, accused of being a traitor

Eventually Pilecki escaped from Auschwitz with two friends in April 1943. They travelled more than 100 miles on foot and Pilecki was shot in the shoulder by a German guard. Back in Warsaw he found that few people knew about the underground organisation he had created in Auschwitz and even fewer were aware of the camp’s role in the mass-murder of Jews.

He wanted to lead an attack on the camp, but the resistance refused. When he discovered that the leaders of the underground he had formed back in the camp had all been shot “he was devastated”. He believed he had failed to convince people that what was happening in Auschwitz was “evil” and that there was a moral imperative to confront it.

It wasn’t until the summer of 1944, when allied forces had landed on the beaches of Normandy and Soviet planes were above Warsaw, that western leaders acknowledged the significance of what was happening at Auschwitz. By then Pilecki had just completed his 10th report on the camp. Five thousand Jews a day were being gassed, so many that the crematoria couldn’t cope and there were giant funeral pyres of corpses.

After the war, Pilecki returned to Warsaw to set up a new underground organisation hoping to liberate Poland from its new communist rulers. In March 1946 he returned to Auschwitz, which had just been turned into a permanent memorial: “He’d come looking for answers but found none.” Instead he turned to writing in order to understand what had happened. But Pilecki was forced to acknowledge “that the horrors of the camp might never be comprehensible, even to a prisoner like himself who had suffered within its walls”.

The following year, he was arrested by the secret police and accused of treason. He was tortured and interrogated more than 150 times in six months: “He told them the truth, he told them lies, he told them what he thought they wanted to hear.”

Eventually he signed a confession and appeared in Poland’s first Soviet-style show trials in March 1948, accused of being a traitor and of “threatening society and our wonderful youth”. During a break in the trial he had a brief conversation with his wife, Maria, telling her: “Auschwitz was just a game compared to this. I’m very tired. I want a swift conclusion.”

Refusing to beg for forgiveness, he told the court: “I tried to live my life in such a fashion so that in my last hour, I would rather be happy than fearful. I find happiness in knowing that the fight was worth it.” He was executed with a bullet to the back of the head on 25 May 1948.

Pilecki died believing he had failed. Yet as this compelling study of his remarkable life shows, he did more than anyone to reveal the true horror of the camp: “It was the allies that didn’t listen.”

• The Volunteer is published by Ebury (RRP £7.99). To buy a copy go to guardianbookshop.com.