“In the spring of 1945,” says the narrator, over bucolic springtime shots of the German countryside, “the allies advancing into the heart of Germany came to Bergen-Belsen. Neat and tidy orchards, well-stocked farms lined the wayside, and the British soldier did not fail to admire the place and its inhabitants. At least, until he began to feel a smell …”

So begins a British film about the Holocaust that was abandoned and shelved for 70 years because it was deemed too politically sensitive. The smell came from the dead, their bodies burned or rotting; or from malnourished, often disease-ridden prisoners in the concentration camp of Bergen-Belsen, near all those thriving German farms. 

As allied troops liberated such camps across what had been German-occupied Europe, the British Ministry of Information’s Sidney Bernstein (who later founded Granada Television) was commissioned to make a documentary that would provide incontrovertible evidence of the Nazis’ crimes.

Bernstein assembled a remarkable team, including the future Labour cabinet minister Richard Crossman, who wrote the film’s lyrical script, and Alfred Hitchcock, who flew in from Hollywood to advise Bernstein on its structure. They set to work on a documentary entitled German Concentration Camps Factual Survey. As they worked, reels of film kept arriving, sent by British, American and Soviet combat and newsreel cameramen from 11 camps, including Auschwitz, Buchenwald, Dachau and Bergen-Belsen. As well as the dead, the footage showed starved survivors and human remains in ovens.

Now, 70 years on, director and anthropologist André Singer has made a documentary called Night Will Fall, to be screened on Channel 4 later this month, telling the extraordinary story of filming the camps and the fate of Bernstein’s project.

Singer tracked down and interviewed survivors who appeared in the original footage. Among them was Anita Lasker-Wallfisch, now 89, who recalled the day British troops arrived at Belsen-Bergen. “It was an unbelievable moment. Suddenly, you hear English spoken. ‘You should remain calm. Don’t leave the camp. Help is one the way.’ That sort of thing.”

“It is difficult to describe,” Lasker-Wallfisch says of her liberation. “You spend years preparing yourself to die and suddenly you’re still here. I was 19. Every British soldier looked like a god to us. It was not what we expected, to be still alive – but there we were.” The sisters settled in Britain after the war. Anita played in the English Chamber Orchestra and became renowned as a solo cellist.

The sisters were exceptional. “I should have known this but didn’t,” says Singer. “Some of those who were liberated remained in those camps for five years after liberation. Often they had nowhere else to go – certainly not to Britain or the US. We didn’t want them.”

Singer also interviews another illustrious Holocaust survivor, a Croatian named Branko Lustig. He was a child in Belsen, so sick at the time of liberation that when he heard a strange noise he thought he’d arrived in heaven to a chorus of angels’ trumpets. In reality, they were the bagpipes played by Scottish soldiers.

Many years later, Steven Spielberg chose Lustig, by then a film-maker, to be a producer for Schindler’s List. Lustig has a theory about why British authorities suppressed Bernstein’s film. “At this time, the Brits had enough problems with the Jews.” By that, no doubt, he means that Britain was dealing with Zionists agitating for a Jewish homeland in the British mandate of Palestine – and seeing the full extent of Jewish suffering would only inflame them.

Singer says he’s already had flak for including Lustig’s theory. “Why the film was scuppered is not very well documented,” he says. “But Branko may well have a point.” Singer points out that in 1945, the incoming Labour government’s foreign secretary, Ernest Bevin, was anti-Zionist and unsympathetic to the foundation of a Jewish state. But he concedes there is no strong proof. “The only documentary evidence we have is a memo from the Foreign Office saying that screening such an ‘atrocity film’ would not be a good idea.” 

Part of the reasoning for that memo, Singer argues, is that the British thought the Germans needed to be nurtured as allies against the growing power of the Soviet Union. But were such compunctions realistic? Would showing the film to postwar Germany have been a propaganda reverse for the British, serving to alienate the Germans and tip the emerging cold war in the Soviets’ favour? Singer doubts it.

No matter. The film, which some have called a forgotten masterpiece of British documentary, was shelved for 70 years. Bernstein died in 1993 and, according to Singer, one of his regrets was not completing his Holocaust documentary.

Footage from his unfinished film, however, proved key to the prosecution of camp commandants at the Nuremberg and Lüneburg trials in 1945. Anita Lasker-Wallfisch recalls testifying at Lüneburg against, among others, Bergen-Belsen commandant Josef Kramer, known as “the Beast of Belsen”. Her evidence was supported by film that contradicted the accused’s defence. “Kramer had said he didn’t have the food to feed his prisoners and that was why they were in such a state. The footage destroyed that,” says Singer. Kramer and other officers from Bergen-Belsen were hanged that year.

Bernstein’s film never got the chance to be as revered as later Holocaust documentaries, including Lanzmann’s Shoah, Resnais’s Night and Fog, and Ophüls’ The Sorrow and the Pity. As Singer explains, an incomplete version was shown at the Berlin film festival in 1984 and on PBS in the US in 1985 under the title Memory of the Camps. Only recently did a team from the Imperial War Museum complete and digitise the picture.

Into the gap left in 1945 by the suppression of Bernstein’s film came another documentary, made by the great Hollywood director and exiled Austrian Jew Billy Wilder. But Wilder’s Death Mills was a hectoring piece of propaganda, keen to indict Germans, while Bernstein and Crossman had attempted to make their film a warning to all of humanity. “Bernstein’s was a work of art by comparison,” says Singer, “mainly because of Crossman’s lyrical script.”

Certainly, German audiences didn’t enjoy Death Mills. Wilder recalls that when it was screened in Würzburg, there were 500 in the audience at the start and only 75 at the end. Whether German Concentration Camps Factual Survey would have had a similar reception is debatable. It would have been anything but easy viewing, not least when Crossman’s script indicted those who lived near Dachau concentration camp but affected ignorance of the barbarism that took place there: “Germans knew about Dachau but did not care.”

Crossman’s script ends with these words: “Unless the world learns the lessons these pictures teach, night will fall. But, by God’s grace, we who live will learn.” Does Singer go along with that? “I wish they had proven to be correct, but since 1945 there have been a number of genocides that have not been stopped by lessons from the past.”

That said, Singer still thinks such deeply upsetting and horrific images should be seen. “I was born on 4 May 1945, so I’m of a generation who knew about these things, but I have sons of adult age who knew little. We need images like this for the new generation.”

So does he think Night Will Fall should be shown in schools? “I do,” he says, “but there’s a strong body of opinion against. It’s seen as too upsetting. But we’re in an age where such imagery is so prolific. I think the imagery in Bernstein’s film and mine, if used in the right context, can only help understanding.

“We can only truly understand the horror of war if we use images like this.”

• Night Will Fall is on Channel 4 on Saturday 24 January at 9pm.

• Read Peter Bradshaw’s review of Claude Lanzmann’s The Last of the Unjust.