The definition of antisemitism will come under the spotlight at a libel trial likely to be heard at the High Court in London later in the year.

Tony Greenstein, the veteran anti-Zionist from Brighton, is suing the Campaign Against Antisemitism for calling him a “notorious antisemite” in a series of articles posted on the organisation’s website.

The CAA relies on the definition of the International Holocaust Remembrance Alliance to justify its description of him.

But Mr Greenstein, who says in court papers he is a “Jew who deplores and condemns antisemitism”, challenges the validity of the IHRA definition.

In a preliminary skirmish on Thursday, the two sides faced each other in a short hearing at the High Court in order to clarify legal issues.

In 2017, Mr Greenstein launched an online petition to ask the Charity Commission to deregister the CAA, claiming its purpose was to limit freedom of speech by attacking as antisemitic opponents of the Israeli state.

The CAA followed that by publishing five articles on its website over the course of nearly a year, which referred to him as “a notorious antisemite”. The first article cited the IHRA definition as the basis for its description, while the subsequent four articles contained links to the first.

Explaining the CAA’s basis for calling Mr Greenstein an antisemite, its counsel Adam Speker said it was “given how often, and how well he is known for, drawing comparisons of contemporary Israeli policy to that of the Nazis; characterising the creation of Israel as ‘racist’; accusing Jews as a people, and Israel as a state, of inventing or exaggerating the Holocaust; and making mendacious, dehumanising, demonising or stereotypical allegations about Jews.”

But Mr Greenstein argued that an ordinary reader would understand the label antisemite to mean “a racist prejudiced against all Jews on the grounds that they were Jewish”.

At this week's hearing, the judge Mr Justice Nicklin had to decide the legal grounds on which the case should be contested, whether calling Mr Greenstein an antisemite constituted a statement of fact or expression of an opinion.

If considered an opinion, it would lower the bar for the defence.

Mr Greenstein’s counsel David Mitchell argued that the CAA had presented it as a statement of fact.

But the judge ruled that it should be regarded as an opinion.