The Labour high command is to be sued by former employees who broke cover last week to criticise the party’s handling of cases of alleged antisemitism in a dramatic escalation of the row engulfing Jeremy Corbyn’s party.

Two of the whistleblowers who featured in last week’s explosive BBC Panorama programme entitled Is Labour Anti-Semitic? – Sam Matthews and Louise Withers Green – contacted the Observer last night to say they had instructed the prominent media lawyer Mark Lewis to act on their behalf because they believed the party had defamed them in its response to their claims. Others who spoke to Panorama are also understood to be considering contacting Lewis to represent them in libel actions.

On the evening the programme was aired, a Labour spokesman said: “It appears these disaffected former officials include those who have always opposed Jeremy Corbyn’s leadership, worked to actively to undermine it and have both personal and political axes to grind. This throws into doubt their credibility as sources.”

Lewis, who works for Patron Law, and is well known for his role in pursuing phone-hacking cases and for representing the family of murdered teenager Millie Dowler, said: “It is incredible that after the programme Labour wilfully attacked the whistleblowers, falsely accusing them of making deliberate, malicious representations, and misleading the public, while also calling them disaffected former officials whose credibility as sources was in doubt.”

He added: “These are very serious libels. Those representing the Labour party have acted in a way that set out to destroy the reputations of the whistleblowers. In their effort to destroy these people they have left it for the courts to decide who is telling the truth. It is ironic that the bosses at the workers’ party have decided to go against the workers.”

Labour has raised complaints at the highest level of the BBC about the Panorama programme, in which eight whistleblowers spoke out, claiming it was slanted and unbalanced. The allegations from whistleblowers included claims that key aides, including the director of communications, Seumas Milne, and the general secretary, Jennie Formby, interfered with investigations.

Both Matthews, the party’s former head of disputes and Withers Green, a former disputes officer, decided to break non-disclosure agreements that they were asked to sign when leaving the party last summer. They insisted they decided to speak out in the public interest.

Matthews, who said he was left feeling suicidal because of the pressures placed on him, said last night: “The Labour party is choosing to ignore the central charges of antisemitism raised by myself and other whistleblowers on Panorama, and instead, they have engaged in a concerted campaign to damage my name. I have instructed Mark Lewis to ensure that the defamation and intimidation of whistleblowers is not allowed to continue.”

Withers Green said: “I am incredibly disappointed that the Labour party has not reflected on or engaged with the troubling situation of inaction on anti-Jewish hatred that fellow whistleblowers and I have raised. This should be a stark wake-up call about our collective duty to root out racism. But instead the party has used its full weight to discredit us, with untrue, libellous statements.”

“I have instructed Mark Lewis to clear my name of the defamatory accusations the Labour party has made against me, and I urge the party to cease attacking its own staff and activists and instead take action on what should be our common enemy: antisemitism.”

Yesterday, during a visit to the Durham Miners’ Gala, Corbyn said there were “many, many, inaccuracies” in the documentary. He said: “The programme adopted a predetermined position on its own website before it was broadcast. We’ve made very clear what our processes [for dealing with anti-Semitism cases] are. Our party members do have the right to be heard if they’re accused of anything and our party staff have a right to be supported and they are supported.”

A Labour spokesman said it will call for the programme to be removed from BBC iPlayer until basic facts are corrected, full and unedited quotes are used, and an apology is issued. On the possibility of legal action, a party spokesperson said the party’s response had been fair: “These are justified statements of opinion. Any claims will be vigorously defended.” The bitterness of the row within Labour deepened yesterday when Unite leader Len McCluskey attacked Labour’s deputy leader, Tom Watson, over his criticism of Formby, who is receiving treatment for cancer. He said in remarks at the Durham gala, which he repeated on Twitter, that Watson was a “fucking disgrace”.

A BBC spokesperson said: “The BBC stands by its journalism and we completely reject any accusations of bias or dishonesty. The investigation was not pre-determined, it was driven by the evidence. The outcome shows the serious questions facing the Labour party and its leadership on this issue. The programme adhered to the BBC’s editorial guidelines, including contacting the Labour party in advance of the broadcast for a full right of reply.”

Today in a letter to the Observer, a group of leading Jewish figures express their “bewilderment and disgust” over the handling of antisemitism by Labour, which is already the subject of an investigation by the Equalities and Human Rights Commission. Howard Jacobson, Simon Sebag Montefiore, Sir Simon Schama, Neil Blair, Tracy-Ann Oberman and Rabbi Julia Neuberger say the EHRC inquiry “is not a matter of housekeeping but a taint of international, historic shame”.

Separately 20 Labour MPs, all members of the Tribune Group, issued a statement expressing their shock at report of the party’s handling of cases and how individual employees have been treated. “We support former employees in speaking out and commend their bravery in doing so,” the MPs say.“The Labour Party has always and always will support whistleblowers in coming forward when they are concerned about wrongdoing. Today a poll reveals that a third of voters believe that Labour is now an antisemitic party. The YouGov poll, for the anti-racist Hope Not Hate campaign, found that 42% of voters believed antisemitism is a “genuine and serious issue” in Labour.

It has also emerged that a group of Labour peers have discussed the idea of resigning en masse should the party not take more decisive action to tackle antisemitism and be more transparent about the party’s disciplinary processes.

Other prominent party figures including former leader Neil Kinnock also criticised the way the party had treated whistleblowers. Kinnock told the Observer: “This is a democratic socialist party and, to us and many others, antisemitism is poisonous. It must be rooted out. There are no excuses for failing to do that, or for attacking those who truthfully identify the sickness, or for denying full information to the National Executive and, therefore, the membership.”