He says Auschwitz is a myth, but he has never set foot in the place, never seen the evidence

The vast, sprawling complex of stone and steel that was once created so lovingly by the cream of Germany's architects and engineers is now an empty, silent mausoleum on the banks of a dark river in southern Poland. They have let the swampgrass grow high among theruins and on a hot summer night when the soughing wind makes it sway, there are those who say that if you listen closely you can hear the foul breath of the beast himself.

The vast, sprawling complex of stone and steel that was once created so lovingly by the cream of Germany's architects and engineers is now an empty, silent mausoleum on the banks of a dark river in southern Poland. They have let the swampgrass grow high among theruins and on a hot summer night when the soughing wind makes it sway, there are those who say that if you listen closely you can hear the foul breath of the beast himself.

Its very size is breathtaking. Standing on a ridge above the floodplain, as I did a few years ago, you cannot see where this Kingdom of Murder - the place to which we have always given the collective name of Auschwitz - begins and ends. It seems to stretch from horizon to horizon, filling an entire, featureless plain between two meandering rivers - the Vistula and the Sola - outside the peaceful town of Oswiecim. Heinrich Himmler, who surveyed the site as he followed the all conquering Wehrmacht, changed its name immediately to Auschwitz. At first the crazed little bureaucrat intended that it should be a barracks for thousands of slave labourers who would work at the factories being built by the chemical giant IG Farben in a futile attempt to turn coal into rubber and petrol. Not a single ounce of either was ever produced.

But later, as Himmler and Germany itself descended into the final madness, and the slaughter of Europe's Jews accelerated through 1943 and 1944, countless thousands of men, women, children and even babies were turned into smoke and ashes. The modest Auschwitz main camp was extended out across the plain to become the gigantic Konzentrationslager known as Birkenau, a place the size of a small city that had only two kinds of buildings - wooden huts for the temporary storage of people, and five massive gas-chamber-crematoria complexes to dispose of them. It became, for nearly three years, the perfect industrialised killing machine.

But David Irving claims almost all of this is untrue. He agrees that many did perish there. But he says they died through disease, hunger and exhaustion, the victims of war, suffering the fate of countless refugees throughout history. There was no planned mass murder, he states categorically. And the numbers who died have been grossly exaggerated. Three million? Four million? Nonsense. Logistically impossible. Nor were there any gas chambers. Just fumigation areas where lice-ridden clothes were treated to prevent disease. And the big crematoria ovens? They were needed to get rid of corpses of people who had died for the reasons given above.

Day after day I watched him at the Law Courts, doing what he likes doing most. Striding back and forth, letting his formidable imagination take over the control of his tongue, working an audience like a craftsman orator. I have known him for nearly 30 years, and he likes nothing better than being the centre of attraction, the man of the moment. The London Law Courts have often been his stage and the atmosphere of a courtroom is like a drug to him and he loves it. But he is neither a fool nor a madman. He may be a show off, an intellectual bully and a man capable of making facts fit his theories, but do not ever doubt his tenacity and his capacity to present an argument, backed by his version of the truth.

Mr Irving, unlike some of the other clowns, fantasists and wealthy anti-Semites who control the growing movement called Historial Revisionism, is - or was - a gifted historical investigator but somewhere, at some point, he lost the plot. For whatever reason, he was swept up into that dark netherworld of Holocaust denial, and he began to glory in the spurious infamy it brought him, strutting about the world, claiming Hitler didn't know about the death camps and that Auschwitz was a Polish tourist trap.

Like a magician producing rabbits from a hat, he produces questions that are disturbing, puzzling, confusing, even bewildering. Remorselessly, he plants tiny seeds of doubt in the minds of even intelligent and reasonable people. Why did the German's build a fully equipped hospital in Auschwitz if they intended to kill everybody? Why did they not destroy the five huge crematoria when they ran before the Russian army? Why did they leave behind thousands of living witnesses? Why were there no holes in the roofs of the "gas chambers" where the Zyklon B pellets were allegedly dropped? The buildings are still there, he roared at one point in his libel trial, and nobody can show me any holes.

On and on it goes. Find some tiny inconsistencies, discover some flaws in eye-witness accounts, present logistical anomalies as Zionist lies - and soon the minds of those who were not even born during the Holocaust are filled with the possibilities that it could all be a lie.

Unlike many of our more celebrated historians of that period, Mr Irving could speak fluent German and didn't spend his time behind a desk in academe. Instead he combed the archives of Europe and, later, Russia, and produced a series of stunning - and thrillingly written - accounts of the Second World War. And he is clever in that he allows some of the horrors to remain. That is what makes him so dangerous. When the neo-Nazi rabble say these things they are easily dismissed. When Mr Irving talks, people begin to listen.

He once turned savagely on me when I suggested that he was part of the Holocaust denial movement. "I have never, never, never denied that great acts of slaughter took place," he roared. "Millions upon millions perished in the war. But we must be very careful to separate myth from truth. And Auschwitz is largely myth."

But Mr Irving is always faced with one glaring truth. He has never, not once, set foot in the place. He has not seen, as thousands of pilgrims - including me - have, the clear evidence that still lies, ruined but intact, on the ground, exactly as it was when the Red Army arrived 55 years ago. And each of his trick questions are, like the crumbling stones of all crazed conspiracy theories, easily answered.

He is right in saying that four million did not die there. But well over a million did. He is lying when he says there are no holes in the roofs of the cleansing areas. There were, and are. I have seen them. The hospital that he boasts about was built for the guards and staff. The crematoria where upwards of 900,000 perished were so large they could not be destroyed, even with hundreds of tons of dynamite that were used. And in their panic, the Germans left behind thousands of mostly dying survivors. And, most graphically of all, years of work by forensic scientists have shown that under the great pools of water at the northern end of Birkenau, is a 30 foot layer of ash - the remains ofthousands of corpses.

Finally, there is the written confessions of the killers themselves, men like Rudolf Hess, the commandant of Auschwitz-Birkenau, which make it clearthat the complex was designed and built by some of the most illustrious industrial corporations of Germany as a complete apparatus for the daily murder of thousands of people.

But you could go on and on answering these absurd fantasies. And this is what the denial merchants want. For many years, the strategy of the authorities, particularly the Jewish authorities, was to ignore the denial movement, hoping it would simply go away. Now, that is no longer possible. Each year it seems to grow in strength, like a virus attacking the heart of a great truth. Now the strategy is to meet it head on, wherever and whenever it raises its head.

To some, the Irving libel trial was seen as the opening of Pandora's Box. The fear was that for the first time in Britain, in a major public arena, the monstrous poison of revisionism had finally been allowed to fly up into the air, perhaps to flourish and grow in the media as the minutiae of the Holocaust is argued by pundits from both sides. And listening to it all - bewildered and confused by the enticing theories of a powerful dealer in fantasy retold as history - a new generation may have been lured into thinking that it was perhaps not as bad as they had been taught. Were there actually no gas chambers? Was Auschwitz largely a myth, not allowed to be challenged? Were six million really murdered, or did they die as victims of war, just like the other 50 million who perished from the shores of France to the steppes of Russia?

Or has something of value emerged from these long weeks in Room 73 of the old law courts in the Strand? I think it has. The systematic slaughter of an entire generation of people who were neither combatants or even enemies, known as the Holocaust and perpetrated by one of the most civilised nations on earth, is unique in our history. There have been many acts of genocidal violence, both before and since, but they did not involve the transport of millions to industrial complexes built and staffed by ordinary German men and women with the single purpose of mass murder over a period of years. It is something that must not only be remembered. It must be studied endlessly, by each new generation, as they try to answer the unanswerable.

And, again uniquely, the final flowering of this catastrophe is still in existence. The ghastly ruined abomination of Auschwitz-Birkenau still exists today, silent and forbidding in the open plains of Upper Silesia. It is only a couple of hours away from Heathrow Airport. It is a place to which we should, perhaps, take all our young men and women, when they are old enough to cope with the unique horror of it, so they may pass their memories to their own children.

The Holocaust Libel action, in all its absurdity, has turned a great spotlight on this terrible place. And as he slinks away like a thief in the night into the oblivion he deserves, the lies of David Irving may have done history a kind of favour.