In an unprecedented move, 127 members of Labour’s Parliamentary staff have signed a letter to the party’s general secretary to express their “dismay at the decision to readmit Chris Williamson MP”.

The Derby North MP was reinstated with a formal warning on Wednesday after a three-person panel decided to reject a recommendation by party officials to refer him to the next stage of Labour’s disciplinary process.

The ensuing backlash has seen 90 Labour MPs and peers signing a letter calling on Jeremy Corbyn to remove the whip from Mr Williamson.

On Thursday 68 Jewish and non-Jewish staff members, including researchers and parliamentary assistants, broke protocol in publishing their own open letter to general secretary Jennie Formby.

In it, they said that the Derby North MP's readmission will help to create an environment where Jewish and non-Jewish staff members “are made to feel unwelcome by his presence at work”.

It read: “We are writing to you as Jewish and non-Jewish members of staff to express our dismay at the decision to readmit Chris Williamson MP into the Labour Party. He has a long pattern of antisemitic behaviour and has shown contempt for the Jewish community on numerous occasions.

“We are asking you to call for an immediate review of this decision, and take steps to implement a truly independent complaints process. One that all victims of racism, antisemitism, sexual harassment, and bullying can have genuine confidence in.”

Also listed were “various actions” taken by Mr Williamson which led signatories to believe that a formal warning was an inadequate response.

They included the endorsements of several Labour members suspended for antisemitism, as well as tweeting a “conspiracy theory article from the website Skwawkbox, accusing the Board of Deputies of British Jews of using antisemitic language”.

A Labour source told Politics Home that a “red line has been crossed here and we can stay silent no longer”.

Mr Williamson was suspended following accusations of a “pattern of behaviour”, referring to inflammatory interventions into the antisemitism row which has engulfed the party.

He was branded a “Jew baiter” in December after signing a petition defending Gilad Atzmon, who was described as an “antisemite who has promoted the works of Holocaust deniers” by anti-racist advocacy group Hope Not Hate.

Mr Williamson also came under fire for saying that Labour had been “too apologetic” over antisemitism.

The Labour Party has been approached for comment.