Gordon Brown is piling pressure on Jeremy Corbyn with a call to “eliminate” anti-Semitism from Labour’s ranks.
The former prime minister says the party owes the Jewish community an “unqualified apology” amid bitter criticism of the way it has dealt with the issue.
In a speech on Sunday, he will call for a “radical change” of policy with automatic expulsion in any case where there is “irrefutable” evidence of anti-Semitism or other forms of racism.
His intervention comes amid widespread anger at the decision last week to readmit left winger Chris Williamson – a strong supporter of Mr Corbyn – after he said Labour had been “too apologetic” in the face of criticism.
The Derby North MP was swiftly re-suspended following a furious backlash, but many MPs remain deeply unhappy at the way the case was handled by the party – which is already under investigation by the Equality and Human Rights Commission.
Labour’s woes deepened with the Board of Deputies of British Jews president Marie van der Zyl accusing the party of giving a “big V sign” to the Jewish community.
She was responding after Jules Rutherford, who is reported to be starting as the party’s head of membership on Monday, retweeted a post describing anti-Semitism claims as “smears” against Mr Corbyn.
And Gordon Nardell, a QC brought in by Labour to deal with anti-Semitism complaints, will leave his role to return to his full-time legal practice in August.
Labour said it would not comment on staffing matters.
Delivering the annual Isaiah Berlin lecture in Hampstead, north London, Mr Brown will say tackling anti-Semitism is about the “moral soul” of the Labour Party.
“We cannot go on ignoring the consequences of the upsurge in hate and hate speech, all too often in the form of sinister, anonymous and untraceable internet trolling,” he is expected to say.
“Opposing anti-Semitism and every manifestation of racism goes to the heart of who we are and what we stand for as Labour.
“It’s about the moral soul of a party, whose most basic goal is a commitment to equality for all – not just for some who suffer oppression – but everyone.
“To fail to act against the abuses we have witnessed runs counter to the very principles of the Labour Party we joined.”
Mr Brown will say that where there is irrefutable evidence of an offence which runs counter to the party’s core principles – such as anti-Semitism – expulsion, rather than just suspension, should be automatic.
An expelled member should a have a right to appeal, but from outside the party, he will say, with an appeals system independent of the Labour hierarchy and appointed in consultation with the Jewish and other communities.
Mr Brown will also call on the party to commit a Labour government to appointing a minister, backed up by an ambassador, charged with combating anti-Semitism nationally and globally.
He will say that it should prepare for government by drawing up a broader strategy for dealing with the problem, including better education in schools and stronger laws against racism in all its forms.
“The Labour Party owes the Jewish community an unqualified apology. But that is only a starting point in rebuilding trust,” he is expected to say.
“We have to call out anti-Semitism for what it is: racism, and, in this case, anti-Jewish racism.
“And I promise that whenever prejudice and intolerance arises, I and whoever I can persuade, are not going to remain silent or stand aside or desert the Jewish community or neglect it or forget.
“It is a promise if ever your voices are silenced, or for whatever reason are not being heard, we will lend you ours.
“It is a promise based on our understanding that while your freedom as a Jewish community depends on all of us; the quality of our freedom depends on yours.”
Labour stressed that Mr Corbyn had apologised for the hurt caused to Jewish people by the anti-Semitism rows that have hit the party.
A party spokesman said: “Jeremy Corbyn and the Labour Party are fully committed to the support, defence and celebration of the Jewish community and implacably opposed to anti-Semitism in any form.
“There has been a deeply worrying rise in anti-Semitism in the UK and across Europe.
“We have strengthened our procedures and increased the rate at which anti-Semitism cases have been dealt with four-fold.
“As recently released data showed, anti-Semitism complaints received over a period of 10 months related to about 0.1% of our membership, but one anti-Semite is one too many. We are determined to tackle antisemitism and root it out of our Party.”