Councillor Ian Jones accused the Labour Party of "covering up wrongdoing" by getting rid of council whistleblowers and replacing them with candidates "who will do their bidding regardless of their ability or record as a councillor or their lack of community activity".
The Tipton Green councillor said a "cabal of anonymous" Labour members in London were intent on facilitating regime change by deselecting "honest, hard working councillors".
He is one of a number of councillors to be elected as Labour in 2016 but who will now stand against the party next month.
Bill Cherrington, who accused Labour of "stifling democracy" when he quit the party in 2019, is standing as a candidate for Reform UK in Princes End.
Mr Jones said he had "reluctantly" resigned as a Labour member after 30 years. "My integrity and honesty towards the people I serve is unquestionable and I have always spoken out when wrongdoing has been identified," he said.
"I will not allow the Labour Party to cover up wrongdoing within the council by getting rid of councillor whistleblowers as seems to be the case at the moment."
He said he had been the victim of a "malicious campaign of smears and false allegations", adding: "It is clear now that the Labour Party’s intentions in these elections is to facilitate a regime change within the council and to remove by deselecting honest hard working councillors within the council.
"It is clear the Labour Party are shovelling in candidates who will do their bidding regardless of their ability or record as a councillor or their lack of community activity.
"I joined the Labour Party because I believed in fairness, democracy and public service. I am leaving because it has abandoned these values."
Mr Jones alleged that Labour was suspending councillors – such as former leader Yvonne Davies – who spoke out against wrongdoing.
He said: "I will fight to get the truth out about the cover-ups within the council.
"I will not go quietly into the night as the saying goes, I will not be coerced, bullied or intimidated to give up my principles and integrity as others seem to have done."
In her departure speech she claimed she had uncovered "widespread misconduct" since becoming leader, including "cover-ups, misuse of public funds, reports being leaked, use of private emails to hide communications, lies being told" and "clear racism".
An independent report into the findings is yet to be published.
The Labour Party has been approached for a comment.