Eight whistleblowers have said how they felt fatally undermined by senior Labour bosses in their attempts to tackle antisemitism, alleging consistent interference in complaints and claiming that one key aide mocked their efforts.

Four of the whistleblowers, including former Labour general secretary Iain McNicol, who left his post last year, have broken non-disclosure agreements (NDAs) to come forward.

Labour antisemitism claims revealed ahead of BBC documentary – live news Read more

One of them, Louise Withers Green, told the Guardian: “People I was working with would show me antisemitic things on their screens and I’d flinch – and then it was deemed not antisemitism or at least the sanctions were lessened to a warning or a reminder of conduct, which means nothing.”

The Labour party has raised complaints to the highest level at the BBC about the Panorama programme. Former officials allege key aides including Labour’s director of communications, Seumas Milne, and the current general secretary, Jennie Formby, interfered with investigations.

Jeremy Corbyn’s personal email address was also copied into leaked email chains, which former officials allege show interference.

Labour robustly denies any interference and said that “former disaffected employees sought the view of staff in the leader’s office, which was complied with in good faith”.

The party and leftwing grassroots group Momentum have launched an online fightback against the programme and its reporter John Ware, as revealed in a leaked strategy document aimed at pro-Corbyn social media “outriders” rather than journalists, first reported by the Huffington Post.

The strategy apparently urges them to draw attention to a Channel 4 Dispatches programme on Conservative Islamophobia and repeatedly to stress that the Labour leader was not antisemitic and the party was dealing with the problem.

It is unclear who authored the advice, which suggested that Corbyn supporters “make sure the truth about Jeremy & Jennie drowns out everything … Under your posts you will be trolled for daring to tell the truth.”

In the Panorama programme broadcast on Wednesday night, the officials allege:

The number of antisemitism complaints received by Labour’s compliance unit increased hugely after Jeremy Corbyn became leader in 2015.

Senior aides contributed to substantial disagreements within the party about what constituted antisemitism.

Aides in Corbyn’s office directly processed complaints on at least one occasion.

Formby attempted to interfere in the process of Labour’s highest disciplinary body, the national constitutional committee, which has the authority to act independently and alone has the power to expel members.

The Guardian understands that the BBC has seen more than 700 emails from the complaints process. In one email from 10 March 2018, the programme alleges that Milne calls for a review of the disciplinary process regarding antisemitic complaints, saying: “Something’s going wrong, and we’re muddling up political disputes with racism ... I think going forward we need to review where and how we’re drawing the line.”

Sam Matthews, the party’s former head of disputes, who has since been threatened with legal action for breaking his NDA, told the programme he interpreted that email as “the leader’s office requesting to be involved directly in the disciplinary process. This is not a helpful suggestion, it is an instruction.”

However, Labour accused the programme of “selective quotation to change the meaning” and claimed Panorama had “deliberately misrepresented this correspondence”. It said the full Milne quote read: “But if we’re more than very occasionally using disciplinary action against Jewish members for antisemitism, something’s going wrong, and we’re muddling up political disputes with racism.”

Another whistleblower, Dan Hogan, who was an investigator on the disputes team, said that, on a number of cases he worked on, people brought in since Formby’s March 2018 appointment “overruled us and downgraded what should’ve been a suspension to just an investigation or worse to just a reminder of conduct, effectively a slap on the wrist”.

One leaked email chain from May 2018 is alleged to show Formby attempting to influence the selection of the panel for the case of Jackie Walker – who was under investigation for alleged antisemitism.

“The NCC cannot be allowed to continue in the way that they are at the moment, and I will also be challenging the panel for the Jackie Walker case,” one email is alleged to say. Corbyn’s personal email was copied into the chain, as well as those of Milne and Karie Murphy, Corbyn’s chief of staff.

Later, referring to the email chain, Formby, a former Unite official, writes – to the same email group: “I’ve permanently deleted all trace of the email. Too many eyes all on my Labour address. Please use my Unite address.”

McNicol told Panorama that the emails should “ring alarm bells across the party” he added: “The NCC was created in a specific way to remove itself from politics and from the political interference. So, to try to interfere politically within the NCC is just wrong.”

Labour told the programme that Formby temporarily stopped using her party email because of concerns a political opponent had access to it.

The party said that the processing of any antisemitism complaints by the leader’s office had been a question of staff resourcing. “These staff were always under the management of GLU staff while carrying out this administrative work,” a spokesman said

In another interview, Mike Creighton, the former head of the Labour disputes team, told Panorama that Milne had approached him for advice about how to handle antisemitism disputes in spring 2016 but alleged he did not take his advice seriously. He advised that antisemitism complaints should be expedited and Corbyn should make a speech on the Middle East affirming Israel’s right to exist.

“He actually laughed at me … I thought he actually wanted to know how we tackle antisemitism within the Labour party. I think what he actually meant to say was: how do we deal with the bad publicity we’re getting?” he said.

A Labour spokesman said the conversation did not take place. “This allegation is false and malicious. Jeremy Corbyn has repeatedly expressed his support for Israel’s right to exist and for a two-state solution ... so there is no reason whatsoever to laugh at any such suggestion.”

Several other officials told the programme that dealing with the scale of the complaints took a severe toll on their mental health. Kat Buckingham, the former chief investigator in the disputes team, said she had a breakdown and had decided to leave the party.

“I couldn’t hold the tide and I felt so powerless and I felt guilty and I felt like I failed,” she told the programme.

Withers Green, a former disputes officer, was signed off with depression and anxiety, and signed an NDA in return for not having to work her notice. She told the BBC she defied the NDA because she would not “be able to live with myself unless I speak up about the horrendous things that I know have been happening”.

A Labour spokesperson said the party was “implacably opposed to antisemitism and is determined to root out this social cancer from our movement and society” and said Corbyn had “proactively addressed antisemitism within the party in direct communications to the party membership, in articles, speeches, videos and interviews”.

The party denied any attempt to intervene in the complaints process. “These disaffected former officials include those who have always opposed Jeremy Corbyn’s leadership, worked to actively undermine it, and have both personal and political axes to grind. It is simply untrue to say that there were any significant number of disagreements about what constituted antisemitism,” the spokesman said.

“The emails … are simply about ensuring the NCC is held accountable for the length of time they take to hear cases and about protecting the party against any successful legal challenge on the basis of perceived bias if the same panel is used in high-profile cases.

“Labour is taking decisive action against antisemitism, doubling the number of staff dedicated to dealing with complaints and cases. And since Jennie Formby became general secretary, the rate at which antisemitism cases have been dealt with has increased fourfold.”

Earlier, Labour released a letter of complaint to the BBC about the programme, saying it “relies heavily and one-sidedly on the claims and allegations of politically partisan anonymous/unnamed sources and Conservative-supporting newspapers and organisations”.