Zarah Sultana has boycotted the first day of the Your Party conference just hours after her co-leader Jeremy Corbyn vowed the pair were united.
Ms Sultana said she will not enter the main conference centre in Liverpool on Saturday in protest at a “witch hunt”.
It is understood that one of her supporters, Kingston councillor James Giles, was denied entry to the conference centre, while other members were expelled on Friday over alleged membership of the Socialist Workers Party.
A spokeswoman for Ms Sultana said: “Zarah met members outside the conference and condemned the recent expulsions.
“This witch hunt is indefensible. We must build a party that welcomes all socialists,” she added. “She will not be entering the conference hall today.”
Already, Your Party has denied claims of a “purge” of members, saying those expelled from the party had broken “clearly stated membership rules”.
A Your Party spokesman said: “These claims are false.
“Members of another national political party signed up to Your Party in contravention of clearly stated membership rules – and these rules were enforced.”
“We’re focused on hosting a democratic founding conference with thousands of members coming together to debate and decide the big issues,” he said. “This is politics outside the Westminster mould: From the ground up, not the top down.”
It comes just hours after Mr Corbyn affirmed that infighting was over in Your Party as he told members: “Last night here in Liverpool, we had a number of events on and Zarah (Sultana) spoke at a great rally, and I very happily and proudly sent a message to that rally – and I was grateful that it was read out to that rally – of support and solidarity.
“I was at a poetry and music event at the Black E, and Zarah sent a message to that.
“As a party, we’ve got to come together and be united because division and disunity will not serve the interests of the people that we want to represent. So that’s the basis on which we launch the party now.”
The fresh commitment to unity came shortly after Mr Corbyn declined to call Ms Sultana a friend when asked during an interview with Sky News. Instead, he said the pair were “colleagues in parliament, and we obviously communicate and so on”.
Two MPs who helped to set up the outfit have also since quit; last week, Iqbal Mohamed said in a statement that he had decided to leave Your Party and continue to serve his Dewsbury and Batley constituency as an independent MP.
Earlier this month, MP Adnan Hussain said he was withdrawing from the party’s “steering process”, citing concerns about factionalism and “veiled prejudice” against Muslims.
Ms Sultana had previously complained she had been subjected to a “sexist boys’ club” after supporters were invited to officially sign up and give the party financial backing. But Mr Corbyn described the move as an “unauthorised email” and just hours later warned members in a statement posted on social media not to sign up via the link.
Mr Corbyn had used Your Party’s supposedly newfound unity to criticise the Labour Party’s structure and bureaucracy.
He said: “I’ve had enough of top-down parties. I spent a lifetime in the Labour Party, mostly fighting Labour Party bureaucracy. I don’t want to repeat that in Your Party. I don’t want to repeat that experience.”
He took the opportunity to urge members to “campaign forever more” for “real socialism”.
Concluding his opening address to the party’s founding conference, the former Labour leader said: “We are here in Liverpool with a huge responsibility on our shoulders to get this thing under way this weekend, to get the branches going, to get the campaigns going and contest the elections in the future.
“This is our opportunity and our time.
“We are going to seize it with both hands, build that party, build that society, and campaign forevermore for real socialism and real social justice.”
This is a breaking news story, more follows…
