David Irving has stated that Hitler knew nothing of the genocide of Europe's Jews. It is a crank outburst here, but a crime in Austria, Germany, Poland and France.

The United Nations General Assembly passed by unanimous consent a resolution on 1 November that "Rejects any denial of the Holocaust as a historic event, either in full or in part". If a historian says - as the leading Holocaust historian of our time, Raul Hilberg, does say - that the number of Jews murdered by the Nazis was 5.2 million rather than the six million, will he be tried before an international tribunal for denying the orthodox version "in part"? Should historic inquiry cease, because the UN and the courts of Austria and Germany have stated their position on the Holocaust? That is no way to suppress fascism. It is fascism.

"Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers."

Article 19 of the Universal Declaration of Human Rights, United Nations General Assembly Resolution, 10 December 1948.

One of my first stories as a reporter for The Observer was a student strike in 1977 at the London School of Economics. Whenever a fellow student spoke against the strikers, they chanted, "No free speech for fascists". It had never occurred to me that free speech should be denied to anyone - fascist, communist or vegetarian. That was 1977, and I have since witnessed free speech denied to both those with whom I agree and those whose views repel me. But my belief in freedom of expression requires me to defend the right of both to speak. Otherwise, what is this free speech I believe in? The freedom to agree?

So, get ready. I am about to defend the right - remember, the right, not the views - of David Irving, who today languishes in an Austrian holding cell for the crime of stating a view that most of us find disgusting. He has stated that Hitler knew nothing of the genocide of Europe's Jews. It is a crank outburst here, but a crime in Austria, Germany, Poland and France. Another anti-Semitic, and much more vicious, Holocaust denier, Ernst Zundel, awaits trial in Germany on a similar charge.

Irving is a historian of the Second World War, who has uncovered important Wehrmacht documents, but defended the Nazis. He supported Zundel in court - not his right to speak, but what Zundel actually said: that the Holocaust was a myth. This places them both beyond the realm of reasonable argument. Their errors could be demonstrated in open debate - as historians have done with Irving's work. Indeed, open debate - without fear of imprisonment and fines - helps to make an open society.

Most of us spoke out in favour of someone who affirmed another genocide. The Turkish government charged the novelist Orhan Pamuk with what can only be called "holocaust confirmation" for asserting that Turkey committed genocide against its Armenian population during and after the First World War. I think Pamuk was right, and I was among many to sign petitions for him. Turkey's citizens should not be obliged to adhere to any orthodoxy. Nor do I believe that Turkey has a right to prosecute those who accuse its armed forces of crimes against the country's Kurdish population. Outside Turkey, this is an easy (and obvious) position to assume. But within the European Community, how many in the literary and human rights worlds who rallied to Pamuk's defence have stood up for the right of two men with whom they disagree to have their say?

I have a free speech hero, a Jewish lawyer in the United States who would never dare deny that Jews were massacred in their millions by Germany. David Goldberger is a law professor at Ohio State University, but in 1977 he worked for the American Civil Liberties Union. The ACLU has an honourable record defending American blacks in the South and free speech throughout the country. Holocaust survivors in 1977 sought to ban a parade by American Nazis through a Chicago suburb. Goldberg represented the Nazis' right to free expression, and he was pilloried for it. But he believed in the constitutional right to express views that he found odious.

Similarly, a conservative Chief Justice of the US Supreme Court, Charles Evans Hughes, wrote in 1931 in the case of Near vs Ohio: "The rights of the best of men are secured only as the rights of the vilest and most abhorrent are protected."

Perhaps nothing is more vile and abhorrent than denying the genocides of our time, whether Armenian, Jewish or Rwandan. But nothing could be more fatal to our rights to speak and to write than for us to deny others the right to deny our dearest beliefs. One day, will it be illegal to assert (or deny) that the United States committed war crimes in Iraq?

The United Nations General Assembly passed by unanimous consent a resolution on 1 November that "Rejects any denial of the Holocaust as a historic event, either in full or in part". If a historian says - as the leading Holocaust historian of our time, Raul Hilberg, does say - that the number of Jews murdered by the Nazis was 5.2 million rather than the six million, will he be tried before an international tribunal for denying the orthodox version "in part"? Should historic inquiry cease, because the UN and the courts of Austria and Germany have stated their position on the Holocaust?

That is no way to suppress fascism. It is fascism.