It was among the most notorious show trials of the 20th century, the prosecution and sentencing to death of Czechoslovakia’s leading communist, who had been arrested in a brutal purge ordered by Stalin.

For decades, events surrounding the revolutionary tribunal that resulted in the execution of Rudolf Slánský, general secretary of the Czechoslovak party, and 10 other defendants was shrouded in mythology – with most visual and verbal evidence apparently lost to posterity.

But an event that has fascinated historians could soon be seen in graphic detail after footage and audio recording of the 1952 trial was found. Hours of film and voice recordings, much of it mould-damaged, believed to cover most of the eight-day procedure were found stashed in metal and wooden boxes – along with millions of classified Czechoslovak Communist party documents – in the basement of a bankrupt former metal research business in Panenské Břežany, near Prague.

The discovery came after Czech historians and archivists lost trace of recordings known to have been made on the orders of the communist authorities in preparation for a propaganda film that was never produced.The material was found by an insolvency specialist, who alerted historians after seeing labels on the boxes that clearly advertised their content.

By strange coincidence, the bankrupt plant occupied the same grounds as a castle which, during the occupation of Czechoslovakia, was used as a residence by the Nazi governor of Bohemia and Moravia, Reinhard Heydrich, an architect of the Holocaust, until his assassination in 1942.

The items are believed to have been removed from the Communist party’s headquarters and hidden after the 1989 Velvet Revolution that ended its 41-year rule in what was then Czechoslovakia.

Historians are unsure whether they were spirited away to ensure their safekeeping with a view to selling them for profit or to conceal evidence about the trial, which is widely considered one of the darkest stains on the party’s record.

The material – consisting of six hours of 35mm black-and-white film and 80 hours of audio – is now with the Czech National Film Archive, which is asking the government for an estimated 12m koruna (£410,000) to fund delicate restoration work that would make it fit for public viewing.

Michal Bragant, the archive’s chief executive, called the discovery “a great opportunity” because it would allow Czech archivists to compile a rare video depiction of Stalin-era show trials, very few of which have available long-form footage.

“The priority is to make the footage safe,” Bragant said. “But it will be even safer once it’s publicly available, because we also need to make the knowledge safe. We still have not learned enough from the 20th century. The more people learn about it and the horror of the show trials, the safer we will be.

“One day, when we can make it available for the public, you will be able to see the facial expressions, people’s movements, the body language and the staged performances. Because this was nothing to do with due process. It was all rehearsed.”

Slánský, who spent much of the second world war exiled in Moscow, was put on trial along with 13 other high-ranking communists in November 1952, accused of “Titoism” and Zionism. The charges were fuelled by Stalin’s wish to purge eastern European communist parties after the Soviet Union’s break with the Yugoslav regime of Josip Broz Tito and his anger that Israel, born four years earlier, had not become a communist state.

The trial had antisemitic overtones, with Slánský and 10 of the other defendants being Jewish. Slánský is believed to have been tortured into a convoluted, rehearsed confession, which he made in court before requesting to be punished with the death penalty.

He was hanged in Prague’s Pankrac prison on 3 December 1952, but was exonerated during the liberalising Prague spring of 1968. Plans to turn the trial footage into a propaganda film were shelved after Stalin died in March 1953.

Martin Vadas, a veteran Czech film-maker, who helped to authenticate the trial materials, said they could fill in large historical gaps. “There is a widespread misconception with the Slánský trial that we already know everything or almost everything,” he said. “The serious problem is that there is no judicial record. The official court file has disappeared. Now [with the film] we can expect to look evil in the eye.”