Jeremy Corbyn is under intensifying pressure to take early action to tackle anti-Semitism in Labour amid fresh claims of abuse by his supporters.
The party has moved to distance itself from a series of pro-Corbyn social media groups after an investigation by The Sunday Times found they contained hundreds of violent and abusive messages.
The claims came as a Labour frontbencher warned there was "real alarm" at the scale of the problem -while comedian Eddie Izzard, who became a member of the party's ruling national executive committee (NEC), said they needed to "make amends" to the Jewish community.
Shadow digital minister Liam Byrne said Mr Corbyn now needed to make good his promise to Jewish leaders to tackle the issue, pointing to the backlog of round 70 cases of anti-Semitism which have still to be dealt with.
He echoed deputy leader Tom Watson in highlighting the case of former London mayor Ken Livingstone - a long-time ally of the Labour leader who remains suspended over comments suggesting Hitler supported Zionism in the 1930s.
"I personally do not think that Mr Corbyn is an anti-Semite, I don't think he has an anti-Semitic bone in his body," Mr Byrne told BBC Radio 4's The World this Weekend.
"But the reality is now that we need action and not simply words. We have got a lot of disciplinary cases stacking up. Mr Livingstone is at the top of that queue."
In a statement, Izzard said the issue had to be dealt with "for the good of the people Labour seeks to represent".
His appointment to the NEC followed the resignation of Christine Shawcroft, a left wing supporter of Mr Corbyn, who quit amid criticism of her opposition to the suspension of a council candidate accused of Holocaust denial.
As the runner-up when she was elected last year, he now automatically takes her place.
"This is a very important time for the Labour Party and we must stamp out completely the stain of anti-Semitism from a minority of members. It has no place in our party," Izzard said.
"We must make amends and repair the damage with the Jewish community as Jeremy Corbyn has promised to do."
The latest developments follow a tumultuous week that has seen fresh recriminations within the party as well as a protest outside Parliament led by Jewish leaders who accused Mr Corbyn of repeatedly siding with anti-Semites.
The Sunday Times said its investigation into 20 of the biggest pro-Corbyn Facebook groups had uncovered routine attacks on Jewish people, including Holocaust denial.
It reported that 12 senior staff working for Mr Corbyn and shadow chancellor John McDonnell were members of the groups which contained more than 2,000 racist, anti-Semitic, misogynistic, violent and abusive messages.
A Labour source said such sites routinely received hundreds of postings a day, most of which were perfectly innocent messages about party policies or events.
Many of the staff concerned were either no longer active on Facebook or were unaware they were members of these groups and had not seen the content highlighted by the paper.
A Labour Party spokesman said: "These groups are not run by the Labour Party or officially connected to the party in any way.
"The Labour Party is committed to challenging and campaigning against anti-Semitism in all its forms. Any complaints of anti-Semitism are taken extremely seriously.
"These are fully investigated in line with our rules and procedures and any appropriate disciplinary action taken."
However the Jewish Labour MP Luciana Berger - who is a regular target for abuse - said she and her staff had gone to the police over the abuse they had suffered from left-wingers, including one email urging her to kill herself.
"Where people indulge in illegal racist activity I will always use the full force of the law to pursue a prosecution," she said in an article for The Sunday Times.
"I will continue to do that even when they are people from the left."