The extraordinary memoir of a Polish resistance fighter who gave the first eyewitness report on the Holocaust to the Allies is to be made into a film by the producer of The King's Speech.

The Story of a Secret State, by Jan Karski, was published in 1944 in the United States. Karski, a devout Catholic, risked his life with the Polish underground and was involved in high-level secret missions to the Polish government-in-exile in London.

A prisoner of the Russians and Nazis, and brutally tortured by the SS, he escaped from Poland and in 1943 was sent to London with a hidden microfilm revealing conditions under the Nazis and in particular the relentless persecution of Europe's Jews by the Third Reich.

As a member of the resistance and to learn the fate of Polish Jews, Karski was smuggled by Jewish underground leaders into the Warsaw Ghetto and infiltrated the Belzec death camp dressed as an Estonian guard. He travelled across occupied Europe to England, and eventually to the US.

Karski personally reported to the Polish Prime Minister in London, General Sikorski, Britain's Foreign Secretary, Anthony Eden, and the US President, Franklin Roosevelt. His description of the systematic annihilation of Jews was met with incredulity.

The producer of The King's Speech, Iain Canning, is believed to have just acquired the rights to the memoir from Penguin, who will publish the book in Britain next month. Film insiders said that Ralph Fiennes, who was Oscar-nominated for Schindler's List, was a likely contender for Karski.

In the postwar period, having been traumatised by memories he described as "my permanent possessions", Karski preferred silence, rather than to relive the horrors. But he revised his book before his death in 2000, expanding it with material he could not reveal during the war.

In the Warsaw Ghetto, Karski said he saw "a cemetery" with living bodies.

"Everywhere there was hunger, misery, the atrocious stench of decomposing bodies, the pitiful moans of dying children." He witnessed two "rosy-cheeked" Nazi youths laughing before taking pot-shots at inmates: "The shot rang out ... Then the terrible cry of a man in agony. The boy who had fired ... shouted with joy." Karski, a man used to Nazi brutality against Poles, remained frozen in shock.

In Belzec, a camp east of Warsaw, he found "squalor" and a "mass of sheer death". Horror-struck, he watched a train being loaded "hermetically", filled with naked bodies "to bursting" - a "quivering cargo of flesh". The floor was covered with quicklime to burn the bodies, "the flesh eaten from their bones," death coming slowly, but taking up to four days.

Before his mission to Britain, Karski agonised over what action the Allies could and would take to stop the murders in the ghetto and camps - if they believed him. "I know that many people will not believe me, will not be able to believe me, will think I exaggerate or invent. But I saw it," he said at the time.

The Jews he spoke to begged him to convey the desire for vengeance and "merciless" bombing and executions of Germans.

Eden presented a report to the War Cabinet, but no direct action was taken as a result of Karski's testimony.

Commenting on whether the Allies could have done more once they knew about the death camps, the historian Andrew Roberts said that the issue was "a huge bone of contention ... amongst historians".

There are "major problems" with bombing, the risk to inmates - "wouldn't look good for Allied propaganda". He also referred to the huge distances that aircraft had to fly and their inability to pinpoint targets.

He added that Karski reported on killings, but did not know about the gas chambers.

"Karski, for all his amazing bravery shown in this wonderful book, was not able to provide ... details of the gas chambers ... It's heart-rending to think so little was done, but then Eden and others argued at the time that they were in fact helping the Jews by trying to defeat the Germans."