Senior Labour figures have insisted the party is committed to driving out anti-Semitism, after Britain's Chief Rabbi suggested Jeremy Corbyn is unfit to become prime minister.
Ephraim Mirvis said Labour's handling of the issue, which has dogged the party under Mr Corbyn, was "incompatible" with British values and that the "soul of the nation" was at stake.
He said the overwhelming majority of Britain's Jews were "gripped with anxiety" ahead of the General Election on December 12, warning "the very soul of our nation is at stake".
The Labour leader is due to give a speech in Tottenham later on Tuesday, where he is expected to speak about discrimination.
Mr Corbyn was set to hail Labour as the "party of equality and human rights" regardless of religion or background when he unveils a document proposing to improve human rights.
Allies of Mr Corbyn's acknowledged Mr Mirvis had raised a serious point, but insisted just a "tiny fraction" of Labour's membership was implicated.
Mr Mirvis received the backing of the Archbishop of Canterbury, the Most Rev Justin Welby, who said the chief rabbi's intervention reflected the alarm felt by many in the Jewish community.
"That the Chief Rabbi should be compelled to make such an unprecedented statement at this time ought to alert us to the deep sense of insecurity and fear felt by many British Jews," he said.
Writing in The Times, Mr Mirvis said "a new poison - sanctioned from the top" had taken root in the Labour Party.
He added: "How complicit in prejudice would a leader of Her Majesty's Opposition have to be to be considered unfit for office?"
Shadow transport secretary Andy McDonald accepted there were issues that had to be dealt with within the party but he strongly rejected the Chief Rabbi's characterisation of Mr Corbyn.
"It is a serious intervention. It is a reminder of the hurt that has been caused to the Jewish community by the instances of anti-Semitism within the party and broader than that," he told Sky News.
"I really do take issue with the conclusions the Chief Rabbi has raised about the character and nature of the party and indeed Jeremy Corbyn who has devoted his life to fight racism of all kinds.
"People come into the Labour Party to fight racism in all its manifestations and it is upsetting, to say the least, to find ourselves trying to deal with these small number of incidents.
"We are a huge movement and this represents a tiny fraction of our membership. Nevertheless it is serious and it has to be dealt with head on."
Two former Labour MPs have now spoken out in support of the condemnation from the Chief Rabbi.
Luciana Berger, who quit Labour in February over the party's alleged anti-Semitic prejudice, said on Twitter it was an "unprecedented and devastating intervention".
And Ian Austin, who quit Labour in February, said it was "utterly shameful. A complete disgrace".
Ms Berger, who is standing as a Lib Dem candidate in the election, said on Twitter: "During the the last meeting I had with Jeremy Corbyn at the end of 2017 I told him about the many public and private Facebook groups that were littered with antisemitic posts which used the Labour leader's name/and photo in their group name.
"Nothing was done about it following our meeting.
"Tonight the party says 'that no one who engages in it (antisemitism) does so in his (Jeremy Corbyn's) name.' But that is exactly what has happened."
And Mr Austin weighed in saying: "It is unprecedented for the Chief Rabbi to have to do this.
"It is heartbreaking to see a party so many of us joined to fight racism and which had such a proud record of fighting for equality reduced to this."