Two of Britain’s biggest trade unions have denounced the treatment of whistleblowers on antisemitism by the Labour leadership, dealing a fresh blow to Jeremy Corbyn.

Unison, the biggest union, and the GMB, the third largest, took the unusual step of publicly calling on the party to protect employees and former employees following condemnation of Labour’s response to fresh revelations about anti-Jewish abuse.

Unison said it had already raised concerns with the party leadership in private over Labour’s attack on whistleblowers and planned to do so again at a key meeting next week.

The unions intervened in the wake of a BBC Panorama documentary last week in which seven former Labour employees broke cover to speak out about their experiences.

They claimed that Mr Corbyn’s senior advisers and allies had repeatedly intervened in cases of alleged antisemitism, including pressure to urge more lenient punishments. Labour denied this, and has complained to the BBC about the programme.

The party leadership was condemned by MPs, peers and some of its own staff over its response to the documentary after it questioned the whistleblowers’ motives, describing them as “disaffected former officials”. The party threatened some of them with legal action.

Unison and the GMB are Labour’s second and third biggest donors, after Unite. The GMB and Unite share responsibility for representing Labour Party staff.

Around 30 current and former Labour officials are understood to be ready to give evidence to the Equalities and Human Rights Commission (EHRC), which is investigating anti-Jewish abuse in Labour.

As the backlash over the party’s reaction to the Panorama programme continued, Unison and the GMB said they were concerned by the response and warned that employees must be free to speak out about wrongdoing without fear of punishment.

A Unison spokesperson told The Independent: “The law is there to protect any employee who raises genuine concerns about the running of an organisation and to protect them from unfair treatment when the issue is raised.

“This should be the case with any employee regardless of who they work for. Unison has already raised its concerns with the party following last week’s broadcast and plans to do so again at next week’s NEC.”

It is understood that Unison contacted Labour about the party’s response to the documentary in the immediate aftermath of the broadcast last week.

A GMB spokesperson said: “Of course people should speak out where they believe there is wrongdoing, and whistleblowers who do that should be protected from reprisals by employers.

“GMB exists for our members, we’re led by them and that’s no different when it comes to the Labour Party as an employer.”

Protests against Labour antisemitism Show all 14 1 /14 Protests against Labour antisemitism Protests against Labour antisemitism Protesters clashed during the demonstration Reuters Protests against Labour antisemitism Members of the Jewish community hold a protest against Labour party leader Jeremy Corbyn and antisemitism in the Labour Party AFP/Getty Protests against Labour antisemitism Protesters hold placards and flags during a demonstration, organised by the British Board of Jewish Deputies for those who oppose antisemitism, in Parliament Square Reuters Protests against Labour antisemitism Hundreds of people gathered in Parliament Square to protest against antisemitism in the Labour Party EPA Protests against Labour antisemitism Labour MP Luciana Berger speaks during the protest PA Protests against Labour antisemitism A protester blows through a shofar during the demonstration Getty Protests against Labour antisemitism Members of London's Jewish community protest in support of Labour leader Jeremy Corbyn outside parliament EPA Protests against Labour antisemitism Labour MP John Mann speaks during a protest against antisemitism PA Protests against Labour antisemitism People protest against antisemitism in the Labour Party as Jewish community leaders have launched a scathing attack on Jeremy Corbyn, claiming he has sided with antisemites ‘again and again’ PA Protests against Labour antisemitism Labour politicians Stella Creasy and Chuka Umunna leave after attending the demonstration Getty Protests against Labour antisemitism A pro-Jeremy Corbyn protester holds a placard during a counter-protest Getty Protests against Labour antisemitism A support of the Labour Party hold up a placard during the demonstration Reuters Protests against Labour antisemitism Jeremy Corbyn supporters during the demo Reuters Protests against Labour antisemitism A protester holds up a sign reading ‘For the many, not the Jew’ AFP

Unite did not directly criticise the party’s response, but a spokesperson said: “All organisations must be required to have an effective and clear whistleblowing procedure. Arrangements must be effective in practice. This requires the promotion of responsible whistleblowing, including awareness of the protections open to those who speak out.”

In a separate development, the GMB branch that represents Labour Party staff is considering putting its members in touch with legal experts to advise them on whether the party could be breaching employment law.

In an email to members, seen by The Independent, branch organisers said: “The GMB has become aware of a number of issues that have been raised by members working for the Labour Party over the past few weeks and months, that is giving rise to concerns about members’ health and well being.

“We are considering offering staff the opportunity to attend a one-to-one drop-in session with one of the GMB’s employment law specialists and if this would be of interest to you, please email me back and we can arrange it Asap.”

Labour's former head of disputes: department's 'mental health went through the floor' while investigating antisemitism claims

In a letter sent earlier this year, law firm Carter Ruck, acting on behalf of the Labour Party, warned Sam Matthews, the party’s former head of disputes and one of the Panorama whistleblowers, that he could face “a drawn-out and costly legal dispute” if he breached his non-disclosure agreement (NDA) with the party.

It accused Mr Matthews of “selectively leaking documents and information to the media in order to generate and feed unbalanced stories which in turn impact unfairly” on Labour.

On Wednesday, Mr Corbyn’s team doubled down on its response to the Panorama programme.

The Labour leader’s spokesman said: “The BBC broke its own editorial guidelines in a spectacular way in the programme, there was no apparent attempt at even-handedness, fairness and impartiality and we want changes to be made to that.

“Something which includes serious factual errors, quite apart from the misleading elements and the extraordinary lack of balance, it’s not appropriate that should continue to be available in that form.”

Asked if anyone should consider resigning in the wake of the show, he said: “What in the BBC? Arguably they should.”

Responding to the trade unions’ comments, a Labour spokesperson said: "The Labour Party is committed to fulfilling our duty of care to our employees, and all employees of the Labour Party have access to an Employee Assistance Programme, which provides 24/7 confidential support to employees, including counselling. This service is widely advertised to employees throughout the organisation.

"The party has legal obligations under data protection legislation to protect confidential data. The letter was sent to one individual several months ago, before the party knew about the Panorama programme. The letter was unrelated to Panorama and it made clear the party was not seeking to prevent any disclosures about antisemitism. The party was not informed by Panorama of the mental health issues experienced by former staff members."

The spokesperson insisted that Labour "would never seek to use settlement agreements which stop the disclosure of discrimination, harassment or victimisation" and would ban such settlements if it was in government.