A bitter dispute between Jeremy Corbyn and Zarah Sultana has cast doubt over their attempt to form a new left-wing party, while providing a major boost to the Green Party.
The split erupted after Sultana launched an online membership portal for the fledgling movement, tentatively called Your Party.
The former Labour leader responded by labelling the site a "false membership system" and warning supporters to cancel any payments made.
He said the portal had been set up without authorisation and confirmed the issue had been referred to the Information Commissioner’s Office.
The fallout sparked an immediate response among left-leaning voters.
The Green Party reported almost 1,400 new members in the 24 hours since the row, pushing its membership above 75,000. Party officials said this represented more than a 10% increase since Zack Polanski won the leadership earlier this month.
Polanski welcomed the growth but remained cautious about future cooperation.
"My door is always open for conversations with Jeremy and Zarah, two people committed to making our country fairer who I really respect. But today I’m focused on growing the movement within the Green Party," he told The Guardian.
In a counter-statement, Sultana accused Corbyn’s allies of excluding her from decision-making, describing the party as a "sexist boys' club".
Corbyn rejected the claim, insisting she "had not been excluded from any discussions" and remained part of a process "rooted in inclusivity and mutual respect".
Sources close to the new movement said efforts were underway to de-escalate tensions, with "serious attempts" being made to identify a way forward.
One insider told The Guardian: "Emotions were running high, but there is an appetite for de-escalation on both sides."