I struggle to believe that Ron Prosor, Israel's Ambassador to the United Nations, does not know that Marine Le Pen is the leader of the French National Front. He struck me as a boorish man when I met him last year, but I did not take him to be that great a fool.

Anyone with a fleeting knowledge of European politics must have heard that the National Front is Europe's most successful far-right party, with roots in the collaborationist Vichy regime, and a history of dismissing the murder of France's Jews as a "point of detail".

Perhaps Prosor forgot. Perhaps the "le Pen" name rang no bells and he was unaware that Jean-Marie le Pen, the National Front's famous and indeed infamous leader, had handed over control of his party to his daughter.

It is possible, I suppose. But even the United Nations, not an organisation noted for displaying squeamishness when it comes to ushering brutes into its conference rooms, knew who she was and wanted nothing to do with her. Marine le Pen invited 100 diplomats to her UN reception. Only Prosor and three others turned up.

The Israeli Foreign Ministry explained that the affair had been the most frightful misunderstanding. Prosor didn't realise whom he was talking to for 20 minutes. When the poor, befuddled man discovered that Marine le Pen was that Marine le Pen, he made his excuses and left. No one in Israel or France believes the official line. Prosor told reporters before his superiors told him to shut up: "We talked about Europe, about other issues and I enjoyed the conversation very much. We flourish on the diversity of ideas". A delighted spokesman for Le Pen added: "There was nothing unclear or ambiguous about our meeting."

Prosor has shown Muslims that Israel will sup with their enemy

I therefore think it is safe to assume that elements within the Israeli right are toying with the idea of doing business with elements within the European far right. They reason that, because fascistic or ultra-nationalist movements have turned their hatred from Jews to Muslims, they are now partners they can embrace. Do I need to tell you what is wrong with this? There are immigrants to Europe and liberals in the Muslim world who have worked out for themselves that authoritarian regimes and movements use antisemitism to manipulate their supporters. Prosor has now shown them that Israel will sup with their most implacable enemies, and led them to think that maybe antisemites have a point after all.

On a diplomatic note, for a foreign government to deliver a propaganda victory to an extremist candidate just before presidential elections in which she may do well breaches all the conventions that govern relations between democracies. Put aside the question of how Israel can now ask France to boycott Hamas, and consider how the National Front used the respectability Prosor conferred on it to reassure wavering voters that Israel can see nothing wrong with a party that targets all Muslim immigrants and children of immigrants rather than the supporters of sectarianism and violence.

Meanwhile, only a diplomat lost in sectarian delusions of his own could believe that the fascist tradition can ever give up on the Jewish conspiracy theory. The far right, and increasingly the far left, needs antisemitism as an organising principle. However much they detest the presence of Moroccans, say, in France, they cannot pretend that poor and marginalised immigrants from the Maghreb control the media, politics, the EU and US.

The world's oldest hatred is old for a good reason. It is a conspiracy theory of everything, which allows initiates to blame all their frustrations and failures on a single demonic cause. It is for this reason that antisemitism would still exist if there were not a single Jew left on the planet.

I met Prosor at a debate in a synagogue about the delegitimisation of Israel. He talked reasonably coherently about how London had become the European capital antisemites look to for inspiration. I know from experience that sections of the liberal-left have taken to parroting racist tropes they deplore in other circumstances to justify their loathings. But then many others do not, and I found it extraordinary that Prosor could not admit that people who were not racists could oppose, say, the expansion of Israeli settlements for good reasons.

"Neutrals will not be convinced by this man," I thought to myself. "He is too much of a blusterer, too cocksure, to win an argument."

Little did I know then that no one would be able to delegitimise Israel quite as well as the Israeli ambassador to the United Nations.