Anti-Semitism within the Labour Party is more widespread than was thought, the Jeremy Corbyn-backing Momentum movement has said.
The group continued that complaints about anti-Semitism cannot be dismissed as right-wing smears.
On Monday, Momentum's National Coordinating Group (NCG) released a statement acknowledging the anger of Britain's Jewish community at the "numerous" cases of anti-Semitism within Labour and the party's failure to deal with them "in a sufficiently decisive, swift and transparent manner".
The move came as Mr Corbyn vowed he would not tolerate any anti-Semitism in the party.
Speaking to Channel 4 News, the Labour leader told that "there is genuine concern" about anti-Semitism within the Party.
He continued: "We are dealing with that concern... by investigating every case that is brought to our attention and if the person has committed an anti-Semitic act, in any way, then they are suspended and usually expelled from the party as a result of that.
"We are not tolerating anti-Semitism in any form in the Labour Party."
On Tuesday, Jennie Formby will take up the role of Labour's new General Secretary, and Mr Corbyn said her first role will be to appoint an in-house lawyer to ensure that procedures for dealing with allegations of anti-Semitism are "absolutely strong and watertight".
Mr Corbyn said he was not aware of allegedly anti-Semitic social media posts made by a Labour candidate for local elections in Kent, Roy Smart, but that he was suspended as soon as they came to light.
The developments came as a Jewish member of Labour's ruling National Executive Committee (NEC), Rhea Wolfson, said she was frustrated at the pace of action on anti-Semitism.
Meanwhile, new NEC member, Eddie Izzard, pledged to "root out the stain of anti-Semitism from Labour", writing in The Mirror: "We must make amends and repair the damage with the Jewish community as Jeremy Corbyn has promised to do."
Mr Corbyn is facing demands to speed up the investigations into a backlog of around 70 complaints of anti-Semitism - including one against Ken Livingstone.
Ex-speaker of the Commons Lord Martin called for an extraordinary members' conference to address the problem.
The former Labour MP told The Guardian: "If you ran a restaurant, and it was dirty and there were cockroaches, you wouldn't get away with saying 'the restaurant down the road is dirty and has cockroaches too'. You would be expected to sort out the problem."
The NCG, which agreed the Momentum statement, includes Christine Shawcroft, who quit Labour's disputes panel and the NEC amid criticism of her opposition to the suspension of a council candidate accused of Holocaust denial.
The statement said: "Accusations of anti-Semitism should not and cannot be dismissed simply as right-wing smears nor as the result of conspiracies.
"Current examples of anti-Semitism within the Labour Party are not only a problem of a few, extreme 'bad apples' but also of unconscious bias which manifests itself in varied, nuanced and subtle ways and is more widespread in the Labour Party than many of us had understood even a few months ago."
While Mr Corbyn's opponents were "opportunistically using this issue as a way to undermine his leadership", this did not reduce the need to challenge anti-Semitism, the statement said.
As well as backing Labour Party initiatives on the issue, Momentum will review its constitution and complaints procedures to ensure they fulfil the group's commitment to "stamping out" anti-Semitism.
The Momentum statement follows a wave of left-wing comment on social media dismissing anti-Semitism complaints as smears from the Labour leader's political opponents.
And it stands in contrast to an open letter to Mr Corbyn from a group called Labour Against The Witchhunt.
The letter - signed by more than 2,500 people including LAW chair Jackie Walker - blamed the allegations on "a cynical alliance between those who wish to deflect criticism of Israel and Zionism, and the right-wing in the Labour Party and the news media, who oppose your wider politics".
Ms Wolfson, who represents constituency Labour parties on the NEC, told BBC Radio 4's Today programme: "There is very clearly a real issue, a huge piece of work that needs to be done in the Labour Party, around modern anti-Semitism and giving people the tools to recognise when legitimate criticism of Israel crosses the line into anti-Semitism."
She added: "Action has been taken. I am frustrated by the pace of action, as I know a lot of people are - there is a heck of a lot more that needs to be done - but things have happened."
Ms Wolfson said she expected the process to enter a "new stage" as new general secretary Jennie Formby takes the reins of the NEC.
"Jeremy has specifically asked her to make this her number one priority," she said. "She starts on Tuesday, so I think we will see a speeding up of these changes."