Left-winger Emma Dent Coad has said she is "devastated" after being blocked from standing as a parliamentary candidate in Kensington.
The former Labour MP - who lost her seat at the 2019 general election - criticised the party and said the selection process is "not fit for purpose".
The campaign group Momentum said the move was a "travesty of justice" while the former Shadow Chancellor John McDonnell urged Keir Starmer to "urgently intervene and ensure fair process" in selections.
Mr McDonnell said the blocking of Ms Dent Coad was "one of the most disgraceful acts I have witnessed in the 50 years I have been a member of the Labour Party".
It comes after some MPs also criticised the decision to prevent Maurice Mcleod from standing as a candidate in Camberwell and Peckham - the seat that will be vacated by the Labour MP Harriet Harman at the next election.
He said at the weekend: "I'm gutted. The party is saying it has no place for an anti-racist socialist like me in Parliament."
It is understood Ms Dent Coad was interviewed by party officials on Friday to make the longlist - the process before members select the constituency's next Labour parlaimentary candidate.
But she was told by the party her application would not be progressed due to concerns about her previous social media posts.
In a statement on Monday, the former Labour MP said: "I am devastated that the Labour Party has blocked me from standing to once again represent my community in Parliament, the community I have spent the last 20 years of my life fighting for.
"I am angry that local members and our local community in Kensington have been denied the opportunity to vote in a free and fair contest, which has been sacrificed for the sake of factional intrigue from Labour officials."
She added: "It is plain as day that the candidate selection process now being run by the Party is being factionally abused and is not fit for purpose.
Former Shadow Chancellor John McDonnell said: "I saw first hand just how committed to the people of Kensignton Emma was when she was an MP.
"That she has been prevented from even standing is outrageous and risks further alienating the Labour Party from the communities we strive to represent.
He added: "These blockings don't just undermine Labour's broad church, they ride roughshod over the rights of our local parties and trade unions. We need Keir Starmer to urgently intervene and ensure fair process."
A spokesperson for the left-wing campaign group Momentum also said Ms Dent Coad "fights tirelessly for her community as a councillor and campaigner, particularly on housing issues so vital to a community scarred by Grenfell".
"This is a deeply damaging episode for the Labour Party in Kensington, and one that local people are unlikely to forget."
A Labour source said: "It's right that the Labour Party expects prospective MPs to uphold the highest standards. Under Keir's leadership that's not going to change"
