A Jewish academic has confirmed he is taking the University and College Union to an employment tribunal.
Ronnie Fraser threatened UCU with legal action in July following its rejection of the widely-accepted definition of antisemitism.
Lawyers acting for Mr Fraser, director of Academic Friends of Israel, wrote to UCU general secretary Sally Hunt alleging the union's stance breached equality laws.
Mr Fraser is a freelance maths lecturer who has been a member of the union for 11 years. His spokesman said UCU's reply to the "serious letter" sent to Ms Hunt had "failed to take the charges seriously and as a consequence, Mishcon de Reya have now filed a claim to the employment tribunal".
The claim alleges that for several years the UCU, and its predecessor, the National Association of Teachers in Further and Higher Education, harassed Mr Fraser by engaging in an "unwanted course of conduct relating to his Jewish identity". Mr Fraser's legal team said the alleged harassment had violated his dignity and created an "intimidating, hostile, degrading, humiliating or offensive environment for him", contrary to equality law.
A spokesman for Mr Fraser said: "The UCU's hostile obsession with Israel and Zionism over the last five years, coupled with its failure to understand contemporary antisemitism, has resulted in the UCU being a place that is not hospitable to Jews". The union's "institutionally antisemitic behaviour towards its Jewish members" had left Mr Fraser with no alternative.
It is thought the tribunal will convene next summer. A spokesman for UCU said the union could not comment while the case was ongoing.