The Jewish Labour Movement told Sky News that it has heard from current and former staff members and party officers on the issue, despite the party being “very keen on silencing those who wish to come forward”.
One whistleblower, former head of complaints Sam Matthews, told Sky News that leader Jeremy Corbyn has “refused” to “take responsibility for what’s happened” to the party.
He claimed that some people still in the party have not come forward to speak out because they are “fearful” of retribution.
Labour has said it is taking “decisive action” against antisemitism and has doubled the number of staff handling complaints and cases.
A spokesman said “we stand in solidarity with Jewish people” and “since Jennie Formby became general secretary, the rate at which antisemitism cases have been dealt with has increased more than four-fold”.
JLM is gathering testimony to submit to the Equality and Human Rights Commission (EHRC), having asked the body to investigate Labour’s handling of antisemitism claims.
Earlier this year, the EHRC decided to launch an investigation into whether Labour had “unlawfully discriminated against, harassed or victimised people because they are Jewish”.
The revelation from JLM comes after former officials told a BBC Panorama documentary that they were undermined in their efforts to tackle antisemitism – something Labour has denied.
Eight people told the programme that Labour’s director of communications, Seumas Milne, and Ms Formby interfered with investigations.
Four of them broke non-disclosure agreements to speak out, including former party general secretary Iain McNicol.
But a party spokesman slammed the programme, saying it “engaged in deliberate and malicious representations designed to mislead the public”.
They added that the officials who spoke to the BBC were “disaffected” individuals who have “both personal and political axes to grind”.
Labour has submitted a written complaint to the BBC.
JLM national secretary Peter Mason told Sky News that the testimonies of the whistleblowers “confirm what we have suspected for a very long time”.
The organisation said more people have come forward to provide testimony in the wake of the Panorama documentary.
Mr Mason said: “What we are also asking at this stage is, given the incredible bravery of those who have already spoken out, that if there are party members, members of staff or members of the NEC [National Executive Committee] who wish to come forward and make clear to the EHRC what is happening inside the Labour party – now is the time.”
JLM came close to disassociating itself from the party earlier this year – and also passed a motion of no confidence in Mr Corbyn’s leadership.
Mr Matthews, who was in his role from 2016 to 2018, said it was his belief that the party is “institutionally antisemitic”.
“There are particular individuals who are responsible for that and ultimately the buck stops with Jeremy Corbyn,” he told Sky News.
Mr Matthews said there was not a “silver bullet” – a single piece of overwhelming evidence – that would lead the EHRC to make such a conclusion.
But he added: “I think that the thing that will be most impactful for the EHRC will be the scale of testimony that I expect them to receive.”
Labour deputy leader Tom Watson wrote to Ms Formby on Thursday following the “appalling” Panorama programme, as he repeated his demand for her to share Labour’s submission to the EHRC.
He also attacked the “deplorable” way in which those former staff who spoke to the documentary were “smeared” by Labour spokespeople.
“Even if some in the party did not want to hear what they had to say, it is unacceptable to attempt to undermine their integrity and characters in this manner,” he wrote.
Mr Watson added: “Panorama painted a picture of a party in which antisemitism is tolerated and allowed to grow.
“It depicted a party with structures completely unfit to deal with the scale of the problem.
“As the political and corporate leadership of this party, the shadow cabinet and NEC must now see the EHRC submission.
“By refusing to share the submission with anyone else you have taken individual responsibility for it but this is a matter for collective not individual responsibility.
“The shadow cabinet and NEC need to know what you have sent to the EHRC in our party’s name, and we need to know urgently.
“Every member of these two bodies is bound by a moral and political responsibility to eradicate antisemitism from the party and restore our standing with the Jewish community.”