Colonel Claus von Stauffenberg is meticulous in his planning. It is 12.37pm on 20 July 1944, and the Nazi officer makes absolutely sure that his briefcase is placed right under the spot where his leader, Adolf Hitler, is poring over a huge map of Europe in his briefing hut at his Eastern Front headquarters, the Wolf's Lair in Rastenburg, East Prussia. >P> But a few minutes later, another German soldier, Colonel Heinz Brandt, accidentally bumps into the briefcase and, thinking it must be in the Fuhrer's way, thoughtfully moves it to the other side of a thick oak table leg. That one, apparently inconsequential act is to have monumental significance.

Moments later, the 2lb bomb contained within the briefcase explodes, kills the four people nearest to it and seriously injures 11 other bystanders. As fate would have it, however, the robust table leg protects Hitler from the worst of the blast and he is but superficially wounded.

The first to emerge, slowly but surely, like a phoenix from the ashes in the aftermath of the explosion, the Nazi leader is triumphant."I am immortal," he boasts to his dazed followers."Providence has given me a sign. I am indestructible." And so a simple table leg alters the course not only of the Second World War but of the entire history of the 20th century. It's a moment to chill you to the core.

This sequence lies at the heart of Virtual History: The Secret Plot to Kill Hitler, a documentary to be broadcast on the Discovery Channel on Sunday. But as undeniably powerful as it is, this footage of the Von Stauffenberg plot may yet prove highly controversial, because these moments were never actually filmed; they have been manufactured for the purposes of the programme. Groundbreaking computer wizardry has brought Hitler back to life in a series of vividly convincing"archive" scenes. In the making of The Secret Plot to Kill Hitler, actors play out key scenes involving Hitler, Churchill, Roosevelt and Stalin. Then, in the editing suite afterwards, the real faces of the four wartime leaders are"magicked" on to the actors' bodies. In addition, the film stock is artificially aged and degraded to make it look like genuine Second World War footage.

Already, criticism has been voiced of Virtual History's method of recreating footage of events that were not actually captured on film at the time. According to Roger Graef, the award-winning documentary maker, the concept of mocking up archive material is"very dodgy":"It is up to the people at the top of broadcasting and the regulators to insist on the flexibility and willingness to back the authentic, in order to resist the easy temptation of putting everything into an artificial box," he says.

Speaking at the launch of Virtual History in the suitably eerie surroundings of the actual Wolf's Lair (a huddle of ivy-clad concrete bunkers deep in the dense forest of north-eastern Poland, hard by the border with Lithuania), the producers are quick to acknowledge that they are potentially stepping into an ethical minefield here.

"If Discovery Channel can do this, then so can North Korea," says Andrew Roberts, the historian and biographer of Churchill who has been acting as one of a bank of historical advisers to Virtual History."It's like nuclear technology - you can't un-invent it. It's just a question of what you do with it.

"There's no technology that can't be debased in the wrong hands. Look at how the internet has been abused by terrorists and pornographers. A totalitarian regime could create false history with this. If China wanted to use computer-generated imagery (CGI) to say that Mao won the Second World War, they could. They could put Mao at the Yalta Conference, telling Churchill which parts of the world Britain could keep."

Roberts is at pains, however, to underline the good faith of the team behind Virtual History, which spent over three years and consulted 15 historians, 250 reference books, 150 historical collections and 2,500 period stills in the preparation of the programme."No responsible historian would take part in a project like this unless the viewers were warned about the technology - and they are very clearly warned at the start of Virtual History.