The Labour party is now engulfed by a problem of antisemitism which it refuses properly to identify let alone effectively confront.

The eruption of anti-Jewish hostility at the Labour conference left decent people aghast. This followed years of escalating anti-Jewish incidents in the party, causing even non-Jewish commentators to criticise Labour’s “serious and growing” problem of antisemitism.

A number of Labour members have now torn up their party card in disgust. Not so Lord Winston, who in a letter to the Jerusalem Post taking issue with my comments on the subject claimed that activists attending conference were not necessarily representative of the party. “I myself”, he added, “seldom need to speak on behalf of Israel in the House of Lords because so many non-Jewish Labour members there are first on their feet to show support for Israel”.

Not everyone shares Lord Winston’s sunny optimism. The Home Affairs Select Committee accused Labour of creating a “safe space for those with vile attitudes towards the Jewish people”. The Labour MP John Cryer said he was shocked by antisemitic tweets by party members coming before its disciplinary panel. Such stuff, he said, was “redolent of the 1930s.”

Last September, Lord Parry Mitchell quit the Labour Party over what he called the “violent anti-Israel views” of the allies of its leader, Jeremy Corbyn. “I think it’s very difficult if you are Jewish and you support Israel to be a member of the Labour Party,” he said.

Clearly, it must be painful for people like Lord Winston and other Jewish party members to try to square their devotion to the Jewish people with their loyalty to the Labour cause.

The truth, though, is that while antisemitism is found elsewhere in British society its major influence and growth are on the left.

A recent survey of political candidates and officials by the Campaign Against Antisemitism found that Labour party office-holders accounted for 61 per cent of cases of alleged antisemitism, nearly eight times higher than the number of office-holders in the second-placed parties.

Last month, a report by the Institute for Jewish Policy Research found that those who were “slightly left-of-centre” or “fairly left-wing” were more anti-Israel than the general population.

The more left-wing they were, the more likely they were to hate Israel. And the stronger that hostility, the more likely they were to hold antisemitic attitudes such as “Jews exploit Holocaust victimhood for their own purposes”.

The main reason is the issue that was largely dodged by the IJPR report — and which also vitiates Labour’s rule change aimed to make it easier to discipline antisemites in the party.

This is the emergence of “Palestine” as the signature motif of left-wing conscience. Anti-Zionism and hatred of Israel are packaged and sanitised as legitimate political positions wholly distinct from antisemitism. This is wrong. Antisemitism is not just one among many different forms of prejudice or hatred. It is a unique derangement of the mind.

Antisemitism is based entirely on lies, distortions and libels, double standards, obsessive hatred, paranoia, the belief in a covert Jewish global conspiracy with an almost supernatural power to do the world harm, and blaming Jews for crimes of which they are not only innocent but are in fact the victims.

Anti-Israel discourse has exactly the same unique and unhinged characteristics. Antisemitism is protean, changing its shape throughout history. The theological and racial Jew-hatred of past centuries has now morphed into an identically unhinged hatred of the collective Jew in the State of Israel.

There’s a still deeper linkage. Jewish self-determination in the land of Israel is an essential component of Judaism, composed as it is of the fusion between the people, the religion and the land. To attack the Jewish right to the land is to attack Judaism itself.

Of course, not all Jews are Zionists. But just as one wouldn’t say the festivals, Shabbat or dietary laws are not essential elements of Judaism because not all Jews observe them, so it’s wrong to say Jewish peoplehood in Israel isn’t an essential element of Judaism just because there are Jews who don’t identify with it.

Not all who demonise Israel are antisemites. Many are simply ignorant of current and historical facts.

But the demonisation of Israel is itself fundamentally anti-Jew. Which is why Labour and the wider left now stand accused of the foulest prejudice of them all.

Melanie Phillips is a Times columnist