A professor at Goldsmiths University has resigned from the University and College Union following its decision last week to reject the European Union's working definition of antisemitism.
Dr Ariel Hessayon, an Oxbridge-educated lecturer in history, said he did not expect to be the only academic to do so. But he said that as a historian who had studied attitudes to the Jewish people, he had no choice but to leave the union.
"Based on my professional expertise, I have no doubt that the politically motivated rejection of the EUMC working definition has antisemitic implications," he said in a resignation statement.
"Accordingly, I cannot in good conscience remain a member of a union that countenances the antics of such extremists; fanatics who seem at best oblivious and at worst disdainful of the consequences of their single-minded obsession: Israel."
He noted that the UCU's efforts would be better spent dealing with "the 'bread and butter' issues that concern the vast majority" of its members.
Dr Hessayon added: "Their relentless promotion of a boycott of Israeli universities – and therefore by extension Israeli academics…has succeeded only in alienating people who while condemning the occupation, nonetheless believe in a just two-state solution as the best way to achieve peace in the region."
The vote to distance the UCU from the 2005 definition was supported by a large majority of delegates at the conference in Harrogate. The decision has been condemned by Jewish groups, with the UCU termed "institutionally racist" by the chief executive of the Jewish Leadership Council
There have also been calls for a formal investigation by the Equality and Human Rights Commission into the decision.
A UCU spokesman said "We don't comment on individual cases."
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