Gordon Brown has said Labour must “immediately and unequivocally” adopt the internationally-agreed definition of anti-Semitism.
In a speech on Sunday, the former prime minister warned the very “soul” of the party he once led was now at stake.
Jeremy Corbyn has been heavily criticised over the summer amid accusations he tolerates anti-Semitism or is even an anti-Semite himself - something the Labour leader denies.
MP Frank Field resigned the party whip over the crisis last week. And today there were further warnings that Labour could split.
This morning the former chief rabbi, Lord Sacks, warned the majority of Jews were now questioning whether Britain was a safe place to bring up their children as a result of the row.
Next week Labour’s ruling National Executive Committee will decide whether to adopt the International Holocaust Remembrance Alliance’s (IHRA) definition of anti-Semitism in full.
The NEC sparked anger among Jewish community groups in July when it endorsed a new code of conduct that failed to adopt all of the IHRA examples.
It was argued that to do so would remove the free speech right of members to criticse the government of Israel.
But speaking at the Jewish Labour Movement’s conference in north London on Sunday, Brown said this claim “cannot be justified”.
“We can criticise specific policies of Israel, we can support the cause of a Palestinian state and we can say with one voice that all racism practiced against Jewish communities is wrong,” he said.
“This declaration is needed now, urgently. Not as some sort of abstract document of philosophy.
“It is needed now to deal with practical threats to confront gathering dangers and on-the-ground realities of very real week-by-week threats to Jewish communities that demand an unequivocal response and unqualified resolve.”
Brown added: “What we say and do on antisemitism raises something that is more profound than a point of policy or procedure.
“It goes to the heart of what we are for. It is about what makes us who we are. It is what in its deepest and truest sense makes us socialists. It is about why we are progressives. It is about the moral soul of our party. ”
Shadow Chancellor John McDonnell told the BBC today he believed “all sides” of the debate over the definition of anti-Semitism would “be satisfied” with the outcome of the NEC meeting.
“From what I’ve heard, it will be resolved, and there will be a balance about acceptance, as people want, but also, exactly about what others have said as well – Lord Sacks himself – that freedom of speech is important as well,” he said.
“The ability to criticise policies that you disagree with, but you do it in terms which are acceptable.”