I have repeatedly defended Jeremy Corbyn against charges of antisemitism. When my neighbour told me the Labour leader was a Jew hater because he had supported the graffiti artist responsible for the hooked-nose, trope-tastic antisemitic mural, I said Corbyn was so anti-racist that he probably didn’t even notice the caricatures – and that he was hopeless at detail, anyway. When photos were released showing him posing with a wreath to commemorate Palestinian martyrs at a cemetery in Tunisia, I said of course he was there – they were killed by an Israeli bomb attack on the Palestinian Liberation Organisation headquarters in 1985 that even Margaret Thatcher’s government condemned. When he refused to sign up to the International Holocaust Remembrance Alliance’s definition of antisemitism, I argued that the definition was problematic, and it was important to be able to say Israel was racist without being labelled an antisemite. And on it went.

But not any more. I still don’t believe (or would like not to believe) that he is an antisemite, but what the Labour leader said at a London conference convened by the Palestinian Return Centre in 2013 is unquestionably antisemitic.

Yesterday the Daily Mail showed footage of Corbyn addressing the conference, on the topic of British Zionists. He mentions an impassioned speech made at a meeting in parliament about the history of Palestine that was “dutifully recorded by the thankfully silent Zionists who were in the audience” (audience members he presumably knew nothing about). So far so bad. But it gets worse. He goes on to say that these unnamed Zionists in the audience “clearly have two problems. One is they don’t want to study history, and secondly, having lived in this country for a very long time, probably all their lives, they don’t understand English irony either … So I think they needed two lessons, which we can perhaps help them with.”

Play Video 0:44 Jeremy Corbyn's 2013 remarks on some Zionists not understanding English irony – video

It is unclear what the irony in question is. But it is irrelevant. To generalise about any race or religion is discriminatory. And if there were ever a clear example of somebody conflating Zionist with Jews, this appears to be it. Let’s play the traditional “swap the minority” game. Instead of “Zionists” let’s make it, say, Muslims or African-Caribbeans or Asians or Irish needing lessons in history or irony. Not nice, eh?

And what exactly does he mean by Zionists who have spent all or most of their lives in this country? Today the party insisted that Corbyn had been quoted out of context and that he had been referring to “Jewish and non-Jewish activists”. Maybe. But it sounds pretty much like he was talking about British Jews to me. And In her 2016 report on antisemitism in the Labour party, Shami Chakrabarti wrote: “Crucially, I have heard testimony and heard for myself first hand, the way in which the word ‘Zionist’ has been used personally, abusively, or as a euphemism for ‘Jew’, even in relation to some people with no stated position or even a critical position on the historic formation or development of modern Israel. This has clearly happened so often over a number of years as to raise some alarm bells in Jewish communities.” She concluded: “My advice to critics of the Israeli state and/or government is to use the term ‘Zionist’ advisedly, carefully and never euphemistically or as part of personal abuse.”

Corbyn 'English irony' video reignites antisemitism row Read more

Meanwhile, Labour’s new code of conduct states that the use of the word Zionism “euphemistically or as part of any personal abuse” may “provide evidence of antisemitic intent”. On both fronts, if Corbyn said the same thing today he would be in breach of his own party’s guidance.

Let’s look closely at the words used by Corbyn: these British Zionists don’t study history, and they don’t understand irony (ironic coming from one of the greatest literalists British politics has produced). In other words, they are uneducated, they have failed to integrate or assimilate, they are outsiders, they don’t belong, they need to be taught a lesson. Sorry, Jeremy, this is the language of supremacism.

I interviewed Corbyn in June 2015 on the day he secured the 35th nomination he needed to make the shortlist for the Labour leadership. At the time I said he had nearly always been proved to be on the right side of history – after the event (embracing Sinn Féin, campaigning for the Guildford Four and the Birmingham Six, fighting apartheid). His indefatigable campaigning for Palestinian rights has also been admirable. But in resorting to such offensive stereotyping of Zionists/Jews, and conflating the two so clumsily/deliberately, he will never be on the right side of history.

• Simon Hattenstone is a features writer for the Guardian