Labour's definition omits key examples of how criticising Israel can be antisemitic, including say Jews are more loyal to Israel than their home country.
But the unions are increasingly showing signs of wanting the party to back the IHRA definition.
GMB General Secretary Tim Roache wrote in a HuffPost article this week: "It is abundantly clear that Labour has to accept IHRA examples of anti-Semitism in full, while agreeing that criticising the Israeli government and supporting our Palestinian brothers and sisters is not being antisemitic."
Unison General Secretary Dave Prentis wrote an article for the New Statesman this week, arguing the same, saying: "This issue is already costing Labour votes, as I saw for myself when campaigning in Barnet this year.
"But worse than that, it's harming Labour's historically close relationship with the Jewish community. And it's costing us the moral high ground from which to oppose all forms of racial hatred and oppression."
