It all began with a question: "Do you deny that the Nazis killed millions of Jews in gas chambers in purpose-built establishments?"

It was a brightly-lit courtroom at the end of a draughty corridor at the High Court in London this week. A quietly spoken QC bent slightly forward from the waist as he asked the question, and a historian answered simply: "Yes". Not any historian, but the keeper - in the eyes of his opponents - of the most horrible lie, the lie of the Holocaust denier.

It was in 1996, while touring Britain in a van to promote his book, Goebbels: Mastermind of the Third Reich, that the revisionist historian, David Irving, became convinced of an "organised international endeavour" to destroy his career.

While visiting several bookstores on his tour he came across another book being sold alongside his own. This 1994 publication, Denying the Holocaust: The Growing Assault on Truth and Memory, was written by a US academic, Prof Deborah Lipstadt, Dorot chair of Modern Jewish and Holocaust Studies at Emory University in Atlanta.

Staff at some of the bookstores made it clear they did not want to sell the Goebbels book and pointed to Prof Lipstadt's work as the reason for their reluctance. Irving's anger at this slight led him to read Denying the Holocaust.

Prof Lipstadt branded Irving a "Holocaust denier" who had distorted historical documents in his publications, including his 1977 work, Hitler's War, to portray Hitler as a "merciful" man. As further evidence of denial, she wrote, Irving had claimed Hitler did not know about the Final Solution of the Jews until 1943 and had dismissed the gas chambers at Auschwitz as "just a legend".

Poring over the pages of her book proved a turning point in Irving's long career as a published historian and has led him into battle against Prof Lipstadt in what is one of the most controversial libel cases ever fought at the High Court.

The issue at the centre of the action brought by Irving against Prof Lipstadt and Penguin Books, which published her work, goes far beyond her allegation that Irving is a disreputable historian. It is also about something more important than financial ruin - the defendants' costs could reach £5 million sterling - if he loses the case.

There is a monumental struggle under way in court 37 about what happens when opinion collides with accepted historical fact. It is about the effect that an alleged Holocaust denier has on future generations as "the lie" spreads around the world and is used by neo-Nazis and right-wing extremists to justify their poisonous beliefs.

It is about whether someone can express an opinion denying the existence of gas chambers in which millions of Jews were systematically murdered, even when the scientific evidence that person relies on to prove that opinion is denounced as "bunk".

The case is expected to last three months. When it began on Tuesday the label of "Holocaust denier" was already "strangling" Irving. His US publisher deserted him after the publication of Prof Lipstadt's book. He had also been banned from Germany and fined DM30,000 over a speech he gave in 1990 in which he said Polish communists built the gas chambers at Auschwitz after the second World War.

In court, with his neat, blue pin-stripe suit and grey hair and insistence on arguing every single detail and fact, even down to the position of a full stop in a historical document, Irving is a consummate historian and performer. He may be facing ruin, but the courtroom and its legal paraphernalia do not intimidate him. He may as well be in the familiar surroundings of a far-right meeting in Germany, for he is not shy of delivering lengthy speeches.

In his opening statement Irving, who is representing himself, spoke of the "waves of hatred" generated against him by Prof Lipstadt and Penguin. He did not deny that the Nazis killed millions of Jews, but it was not a systematic programme of annihilation.

In a grandstanding performance, Irving rejected the label of "Holocaust denier" as he thumped the table in front of him with his fist. "It has become one of the most potent phrases in the arsenal of insult, replacing the N-word, the F-word, and a whole alphabet of other slurs . . . It is a verbal yellow star," he said.

Enter Mr Richard Rampton QC, representing Prof Lipstadt and Penguin. Irving challenged the defendants that it was not enough for them to show he had misrepresented history, but that he had done so "perversely and deliberately" in order to deny the Holocaust and show that Hitler had no knowledge of it.

Switching from one photocopied page of Hitler's War to another and then to the pages of Goebbels: Mastermind of the Third Reich, Rampton assured Irving that he was not in the business of using "Perry Mason" methods to trap him. He simply wanted the truth.

Turning on his opponent with the grace of a cunning fox, Rampton put it bluntly: "He is not a historian at all, but a falsifier of history . . . He is a liar." Rampton delved into the chronology of Irving's "sea-change" from believing in the Holocaust to outright denial that it ever happened.

In the first edition of Hitler's War, which was published in 1977, Irving accepted the Holocaust as historical truth. But by 1991, when the second edition was published, he had expunged all references to the Holocaust.

Irving was also spreading his new message in public in a speech in Calgary: "I don't see any reason to be tasteful about Auschwitz. I say quite tastelessly, in fact, that more women died on the back seat of Edward Kennedy's car at Chappaquiddick than ever died in a gas chamber at Auschwitz . . . I'm going to form an association of Auschwitz survivors, survivors of the Holocaust and other liars, or the ASSHOLS."

What happened to Irving during the 14 years between publications? What happened was Fred Leuchter, a US expert on execution techniques. Irving met Leuchter in 1988, and Leuchter told him about his report on the use of hydrogen cyanide at Auschwitz.

In the report Leuchter revealed that the concentration of hydrogen cyanide required to kill humans was about 22 times lower than that required to kill lice. Subsequently, Leuchter found very small traces of hydrogen cyanide in the gas chambers and relatively large traces in the delousing areas.

However, Leuchter's report alleged, despite what he had written in the body of the report, that the gas chambers at Auschwitz could not have been gas chambers after all because of the low levels of hydrogen cyanide he had found there.

Why did Irving believe Leuchter's report? Rampton asked. "The answer must be that he wanted it to be true. After all, if the Holocaust never happened, then Hitler cannot have ordered it or known about it: thus, as Mr Irving himself said of the second edition of Hitler's War, `You won't find the Holocaust mentioned in one line, not even in a footnote. Why should you? If something didn't happen, then you don't even dignify it with a footnote'."