Senior party members in Birmingham said they were "alarmed" at the selection of James Cunningham as the candidate to replace the retiring Adrian Bailey in West Bromwich West.
It is understood that Mr Cunningham the subject of a number of outstanding complaints made to the Labour Party relating to allegations of bullying.
In a letter to Labour general secretary Jennie Formby seen by the Express & Star, members question Mr Cunningham's "character, integrity and general suitability" to be a Labour candidate, accusing him of "bullying, lying and manipulation" over a number of years.
They also allege that he encouraged Labour activists not to campaign for the party’s candidate Preet Kaur Gill in Edgbaston in the 2017 General Election.
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Mr Cunningham is a Labour community organiser in Birmingham and a former member of the Alliance for Workers Liberty, a Trotskyist group which aims to end capitalism.
One official complaint from a Birmingham-based CLP was lodged against Mr Cunningham in 2018, which they say has not yet been dealt with by the West Midlands regional office.
Another complaint against Mr Cunningham from the same year alleges that his "bullying" behaviour affected the mental health of a Labour candidate.
Today I had a shortlisting interview for West Brom West. I got lovely positive feedback about how good my interview was and how great I came across but they gave it to a man that worked for WM Regional office as a ‘community organiser’ 🤷🏻♀️— Nicky Brennan 🔥 (@nickyebrennan) November 9, 2019
It is believed that none of the complaints against him have led to a full investigation, as the Labour Party did not consider them serious enough to reach the threshold for a breach of party rules.
A senior Birmingham Labour Party source told the E&S: "Its a disgrace that a seat that once had the formidable Betty Boothroyd now has a Labour candidate who bullies and intimates female Labour Party members.
"The voters of West Bromwich West deserve better."
The Labour Party did not respond to a request for a comment.
Having first won the seat in a 2000 byelection, 73-year-old Mr Bailey held West Bromwich West for Labour in 2017 with a majority of 4,460. He announced his plans to stand down two weeks ago.