David Irving, the disgraced historian, was jailed for three years in Austria today after pleading guilty to criminal charges of denying the Holocaust.

The jail sentence was imposed despite a last-minute about turn by the author in court: he conceded that he had been mistaken when he claimed in two speeches in 1989 that gas chambers at Auschwitz were a "fairytale".

Irving appeared visibly stunned by the verdict. As he walked out of court after the seven-hour hearing to begin his sentence he could only say: "I’m very shocked and I’m going to appeal."

"Stay strong, stay strong, good luck to you," one onlooker in court shouted to him in English.

The three judges and eight jurors were not impressed by Irving's change of heart in the courtroom. Passing sentence, the presiding judge described him as a "falsifier of history dressed up as a martyr" who had denied the "greatest crime of the 20th century".

Although it is substantially shorter than the maximum sentence of ten years imprisonment under Austria's 1946 Banning Law, the sentence has raised concerns that Irving will become a martyr among his followers on the far Right.

Before giving testimony in Vienna the British writer denied that he had ever written a book specifically about the Holocaust. He said that he now accepted that millions of Jews were indeed murdered in Nazi death camps.

"I am not a Holocaust denier. My views have changed," he said outside court. "History is a constantly growing tree: the more you learn, the more documents are available, the more you learn, and I have learned a lot since 1989.

"Yes, there were gas chambers," Mr Irving added. "Millions of Jews died, there is no question. I don’t know the figures. I’m not an expert on the Holocaust."

The Banning Law makes it an offence to publicly diminish, deny or justify the Holocaust. Irving has been held without bail since November on charges stemming from two speeches he made to Austrian rightwingers in 1989.

Six years after a High Court libel trial that demolished his reputation as a historical researcher, the case has once again made Irving a cause célèbre among European neo-Nazis.

His lawyer says that he received 300 pieces of fanmail a week during his incarceration and the Vienna courthouse was under high security to head off neo-Nazi protests.

The 67-year-old arrived at the courthouse in the same blue pinstripe suit he wore to the High Court trial and, despite being in handcuffs, clutching a copy of his notorious 1977 book, Hitler's War, which claimed that the German leader had no knowledge of the "Final Solution".

During his three months in prison awaiting trial he has been working on his memoirs, under the working title Irving's War.

Once in the dock, and with his guilty plea out of the way, Irving addressed the court in German "I made a mistake when I said there were no gas chambers at Auschwitz," he said.

But he insisted that he never wrote a book about the Holocaust, which he called "just a fragment of my area of interest." "In no way did I deny the killings of millions of people by the Nazis," Irving testified.

Irving has been in custody since his arrest on November 11 at a motorway service station on charges stemming from two speeches he gave in Austria in 1989.

He was accused of denying the Nazis’ extermination of six million Jews after he said: "74,000 [Jews] died of natural causes in the work camps and the rest were hidden in reception camps after the war and later taken to Palestine, where they live today under new identities.

“I stand by what I said, there were no gas chambers in Auschwitz.”

The state attorney’s office said the 1989 remarks were "a dangerous violation of freedom of speech". Irving's lawyers dismissed the law as outdated.

The trial comes amid fierce debate over freedom of expression in Europe after the publication of caricatures of the Prophet Muhammad that has triggered violent demonstrations in the Islamic world.

Although Austria's Banning Law is often applied - charges were brought against 724 people in 2004 - it rarely produces a prison term, and many Austrians fear that Irving could become a far-right martyr if he is jailed.

In 2000, the historian sued an American Holocaust scholar, Deborah Lipstadt, for libel in the High Court in London, but lost. Mr Justice Gray, the judge in that case, ruled that Mr Irving was indeed "an active Holocaust denier ... anti-Semitic and racist".

Dr Lipstadt said tonight after the sentencing: “I don’t think anything’s going to change him. He should have been met by the sound of one hand clapping. The one thing he deserves, he really deserves, is obscurity.”

Lord Janner, chairman of the Holocaust Educational Trust, said he was pleased at Irving’s conviction. “It is the conviction and not the sentence that matters. It sends a clear message to the world that we must not tolerate the denial of the mass murders of the Holocaust,” he said.

Gerald Howarth, Conservative MP for Aldershot, said that the Austrians “must have taken leave of their senses” to jail Irving.

Mr Howarth, a council member of the Freedom Association, said: “I think he is the victim of Austria’s collective self-flagellation. After all, it was their President Waldheim who turned out to have a rather undistinguished war and I think they have been breast-beating ever since.”

Richard J Evans, the Cambridge history professor whose forensic demolition of Irving's research was key to that defeat, also criticised Austria's decision to charge Irving, which he said risked making him a martyr to freedom of speech.

"I think the media circus that we see in operation now, with hundreds of reporters and TV and radio crews crowding around the courtroom, shows how counter-productive it all is," Professor Evans told Times Online.

"Irving was virtually forgotten before this trial came up and it's simply drawing unjustified attention to a discredited figure."

Roger Boyes of The Times, who was at the Vienna court today, said that Irving entered the courtroom "with a swagger" but was soon put under pressure by relentless cross-questioning from the chief judge, who forced him to apologise for pretty much every view he had expressed over the past 20 years.

Boyes added: "It's becoming like a free speech seminar. You've got al-Jazeera here, you've got Jyllands-Posten, all the people affected by the cartoon war. Everyone one is asking why it's taboo to attack the Holocaust but not to attack the Prophet Muhammad.

"But the case was fought on the detail of what he said, whether he's really retreated, whether his apologies are really worth anything, whether the judge jury could believe Irving's remorse."