Two members of the Labour Party in Peterborough have resigned, citing the party's "bullying and incompetence" in dealing with anti-Semitism.
Richard Ferris and Matthew Mahabadi had earlier criticised the selection of a local candidate who allegedly suggested the Holocaust was a hoax.
In a joint statement they said blowing the whistle in March had exposed them to "alienation and ignorance".
Labour group leader Shaz Nawaz described the move as "disappointing".
In their resignation statement, the councillors said it was a "privilege" to serve the city, but that it was "tiring, stressful, soul-crushing at times".
They were critical of the city's leading Conservative administration, describing them as "grown men acting like children".
In March Mr Ferris, the party's deputy leader in Peterborough and a councillor for Park Ward, and Mr Mahabadi, who represents East Ward, were openly critical of a decision to allow Alan Bull to stand for election.
Mr Bull was then suspended over a 2015 Facebook post in which he appeared to question the historical basis of the Holocaust.
He later apologised, saying the post was sent to a "closed group" of friends to "invite discussion and debate".
In an article posted in the New Statesman, Mr Ferris and Mr Mahabadi criticised the Labour Party over its "failure locally and nationally to deal with an extremely serious case of unapologetic anti-Semitism".
"It seems we'd let a cork out from the bottle, across the party," they wrote.
'Enough is enough'
The two men set up a group called Labour Councillors Against Anti-Semitism the same month, saying they were "sick and tired of the lethargy in taking action".
"We have faced pressure, bullying and alienation by a powerful minority of 'militant' elements of our own Labour Party," they said, "to shut up, pipe down, keep schtum about the problem of anti-Semitism in the Peterborough Labour Party.
"Enough is enough."
The council's leader, Conservative John Holdich, said the two were "victims of weak inexperienced leadership locally", and described them as "not only negative but destructive".
Their seats will not be contested as it is less than six months until the next city council elections, which take place in May.