David Irving claims to have made mistakes. Last week the revisionist historian pleaded guilty in a Viennese court to charges of denying the Holocaust. “I made a mistake when I said there were no gas chambers at Auschwitz,” he confessed, referring to a 1989 speech that he gave in Austria. He did his best in court to imply that he had changed his views.

Irving’s former “mistakes” did not arise from deficient knowledge or carelessness. They were not the product of innocent misconception or misunderstanding. As I discovered when interviewing him shortly before his arrest in Austria last November, Irving’s “mistakes” resulted from his own prejudice — his desire to distort facts to suit his own malevolent agenda fuelled by his virulent anti-semitism.

To some a three-year prison sentence may seem harsh and the curbing of free speech — however odious the speaker — as regrettable. Irving plans an appeal. So, too, do the Austrian prosecutors seeking to lengthen the three-year prison term which they claim was too lenient in the light of “Irving’s special importance to right-wing radicals”. One thing is certain: the Austrian verdict makes it clear that Irving’s mistakes were neither innocent nor genuine nor forgivable.

I first approached Irving in 2004 while researching a biography of Nick Griffin, the Cambridge-educated chairman of the British National party who has striven to become the acceptable face of British fascism. I met Irving at his rented flat in Mayfair, central London. He had just returned from a speaking tour in America designed to sell his back catalogue of 30 titles to the neo-Nazi white supremacists of middle America.

Surrounded by the remnants of his suitcase, Irving, 67, stood in a crumpled shirt, somewhere between scruffy and dishevelled. There was a shifty feeling about him and his eyes had the sort of manic expression that Jack Nicholson captures so well on screen.

Shaking hands with Irving is rather unpleasant. He has spent a lifetime shaking hands with surviving Nazis, the children of Germany’s Nazi rulers and new supporters of national socialism. Pressing the flesh on his big thick fingers I was therefore only a few handshakes away from the hands of the concentration camp guards that dropped Zyklon B pellets into the gas chambers and from Adolf Eichmann, Heinrich Himmler and Adolf Hitler himself.

We lunched amid the mahogany panelling of his favourite restaurant, directly opposite the flat, shared with Bente, his 41-year-old Danish companion, and Jessica, their 12-year-old daughter. At first blush he resembled the broken and bankrupt figure I had expected — quiet and a little pathetic. Then he started talking animatedly about Jews. For the next three hours they were central to every facet of his conversation. I later counted the number of times he used the words Jews or Jewish on my tape: 317. Irving’s paranoid world is full of conspiracy, plot and intrigue. He believes that he is a target for Jewish groups everywhere.

He complained of the constraints of “Jewish advertisers” on the British press and particularly of the late editor of the London Evening Standard, Stewart Steven, whose parents fled Nazi Germany in 1936 taking him with them: “He banned my name from ever being printed in the paper.”

Irving moved on to talk sympathetically about Julius Streicher, one of Hitler’s earliest followers, who published the notorious anti-semitic newspaper Der Stürmer from 1923 to 1945. He was hanged at Nuremberg in 1946 for his role in inciting the extermination of Jews: “Der Stürmer was very clever,” Irving smiles in great satisfaction. “The quality of the draughtsmanship of cartoons was spectacular. I’ve got a series of them on my website.

“There’s the postcard of the Jewish man in the street handing sweets to little children.” Like a grotesque sideshow, Irving distorted his face and lowered his voice, affecting a German Jewish accent: “ ‘Here little girl, here’s a little sweetie for you. I have more at home’. Viciously drawn, but spectacular, excellent draughtsmanship. Not a capital offence of course. But poor Julius was hanged. Today he wouldn’t even get a suspended sentence.”

Until last week’s convenient act of penitence in an Austrian courtroom, Irving has continually denied that the Holocaust ever happened. His book, Hitler’s War, asserted that before 1943, Hitler had no knowledge of the Holocaust. After this biography Irving’s politics and his historical analysis became increasingly extreme. In his 1987 biography of Churchill, Irving accused the British war leader of being debauched, a cowardly alcoholic and a corrupt warmonger who was a slave to “international Jewry”.

Since the mid-1980s he has consistently suggested that the Holocaust is a fabrication and that there was no systematic killing of Jews in Nazi Germany. He has followed the Dr Goebbels line: “If you tell a lie big enough and keep repeating it, people will eventually come to believe it.” Fortunately, people do not believe him.

In March 2000 he lost a three-month libel trial when he sued Penguin Books after it published a book by the American historian, Deborah Lipstadt, which called Irving a denier of the Holocaust, a Hitler partisan, an anti-semite and a right-wing extremist. Professor Richard Evans, the Cambridge historian, said in evidence that “Irving cannot be trusted anywhere, in any of them, to give a reliable account of what he is talking or writing about”.