Mr Irving faces a maximum sentence of ten years in jail under strict Austrian laws making it an offence to publicly diminish, deny or justify the Holocaust. He has been held without bail since November on charges stemming from two speeches he made to Austrian rightwingers in 1989.
But, speaking to reporters as he was escorted into the courthouse, Mr Irving said that he was no longer questioning the deaths of millions of Jews in Nazi death camps during the Second World War.
“I am not a Holocaust denier. My views have changed,” he said. “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 67-year-old arrived at the courthouse wearing a blue business suit and, despite being in handcuffs, clutching a copy of his most notorious book, Hitler’s War. During his three months in prison awaiting trial he has been working on his memoirs, under the working title Irving’s War.
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Austrian authorities have ramped up security around the trial, closing down the upper gallery of the courthouse to prevent protesters from throwing objects. Mr Irving has received hundreds of pieces of fanmail a week since his arrest at a motorway cafe on November 11, and police fear that hundreds of neo-Nazis could turn up to support him.
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 Mr Irving could become a far-right martyr if he is jailed.
Mr Irving had tried to win his provisional release on ?14,000 bail, but a Vienna court refused, saying that it considered him a flight risk. He said today that he had “no choice” but to enter a guilty plea, but said that it was “ridiculous for me to be standing here on trial for something I expresed 17 years ago”.
In 2000, Mr Irving sued an American Holocaust scholar, Deborah Lipstadt, for libel in the High Court in London, but lost. The court ruled that Irving was indeed “an active Holocaust denier … anti-Semitic and racist”.