A Labour MP facing disciplinary action has branded the probe into his conduct a “farce and a disgrace”.
Ian Austin’s lawyers have accused Labour’s general secretary of failing to “observe the most rudimentary principles of natural justice”.
They say Labour is using the probe to silence his “legitimate, honestly-held criticisms of Mr Corbyn’s failure to address the scourge of anti-Semitism”.
Labour has dropped a similar investigation into MP Margaret Hodge.
The party launched an investigation last month into alleged “abusive conduct” by Mr Austin after reports of a heated clash with Labour chairman Ian Lavery over the party’s code of conduct on anti-Semitism.
It later emerged that complaints had also been made against him by Labour MP Chris Williamson and by Mr Corbyn himself, although the Labour leader had not made his complaint formally.
In a letter to Labour’s general secretary, Jennie Formby, Mr Austin’s solicitors, Hamlins, call on the party to drop the investigation or reveal the substance of the complaints against him and the rules he is alleged to have broken.
The letter calls the party’s disciplinary procedures “Kafkaesque”.
It echoes a letter sent by Dame Margaret Hodge’s solicitors to Ms Formby ahead of the party’s decision to drop its disciplinary procedures against her.
Dame Margaret was said to have called Mr Corbyn an “anti-Semite” and a “racist” in a heated exchange with the Labour leader last month.
“I stand by what I said,” she told the Evening Standard.
“Ian acted in a very similar way to me and it is obvious that the case against him should now immediately be dropped.”
She accused the Labour leadership of using trumped-up disciplinary actions to purge critics of Mr Corbyn.
“I have absolutely no doubt that there are those in the leadership who want to get rid, whether it is through de-selection or disciplinary action, of any opposition,” she told the Standard.
“The new style of politics is bullying and intolerance, not gentle and inclusive.”
Mr Corbyn has been criticised by Jewish community leaders and some of his own MPs over what they see as his refusal to take allegations of anti-Semitism in the party seriously enough.
The row centres on the Labour Party’s code of conduct – which includes an internationally recognised definition of anti-Semitism but omits four of the 11 examples of “contemporary anti-Semitism”.
Labour says it has “expanded and contextualised” the International Holocaust Remembrance Alliance examples that it has used to provide “legally sound guidelines that a political party can apply to disciplinary cases”.
And Mr Corbyn has insisted anti-Semites have no place in the Labour Party and acknowledged the party has not been quick enough to deal with those who have spread “poison” in the party’s name.