Jeremy Corbyn is seeking legal advice after thousands of supporters of his new political party received an email urging them to become paying members, which he said was sent without authorisation.
The email was circulated on Thursday morning, inviting supporters of the party, currently operating under the interim name Your Party, to sign up through a new portal with monthly or annual subscription fees.
Within hours, Coventry South MP Zarah Sultana, who co-founded the party with Corbyn in July, said more than 20,000 people had already joined as members, bringing in what could amount to more than £1 million a year in income.
Corbyn responded with a statement on X, co-signed by five of the party's MPs, urging supporters to ignore the message and cancel any direct debits.
"This morning, an unauthorised email was sent to all Your Party supporters with details of a supposed membership portal hosted in a new domain name. Legal advice is being taken. That email should be ignored by all supporters," he wrote.
In a joint letter, the MPs said the agreed process for building the party had been set out on 1 August, and that official membership would only open once the structures for the inaugural conference were in place.
They said the proper membership portal would be available at yourparty.uk later this autumn.
Sultana issued a lengthy response rejecting Corbyn's statement and accusing her colleagues of excluding her from decision-making.
The party's launch has been marred by reports of divisions and infighting among its founders.
"After being sidelined by the MPs named in today's statement and effectively frozen out of the official accounts, I took the step of launching a membership portal so that supporters could continue to engage and organise," she said in a post on X. "This was in line with the roadmap set out to members on Monday and is a safe, secure, legitimate portal for the party."
She said her decision was motivated by the need to safeguard grassroots involvement.
"My sole motivation has been to safeguard the grassroots involvement that is essential to building this party," she said.
“Unfortunately, I have been subjected to what can only be described as a sexist boys' club: I have been treated appallingly and excluded completely. They have refused to allow any other women with voting rights on the Working Group, blocking the gender-balanced committee that both Jeremy and I signed up to."
Sultana also raised concerns about the financial control of the new organisation.
"I do not believe members will accept Karie Murphy and her associates having sole financial control of members' money and sole constitutional control over our conference. This undermines the democratic principles we agreed to uphold," she said, referring to a longtime Corbyn ally.
She pointed to an agreement that funds raised by the party would be managed by MOU Operations Ltd until the founding conference, when they would be transferred to a democratically established entity.
"Every penny raised so far has gone to MOU Operations, and this continues to be the case within our new membership portal. This, to be clear, is members’ money - and our members must decide how it is spent."
Sultana called on Corbyn to meet her and agree to publish all structures and processes to restore confidence.
"This will restore hope for our members and ensure nothing like this can ever happen again. This party is more important than any one person, and we all owe it to the movement to deliver a truly democratic and socialist party," she said. "No stitch-ups, no coronations: the members must decide."
The dispute comes just weeks after the launch of the party, which Corbyn and Sultana announced in July as a new left-wing alternative to Labour.
More than 800,000 people had signed up as supporters by late August, according to party figures, with backing from the Independent Alliance of six MPs and dozens of former Labour councillors.
Your Party has positioned itself to the left of Labour, pledging wealth redistribution, nationalisation of key industries, investment in social housing and opposition to NHS privatisation.
It has also adopted a strongly pro-Palestinian stance, promising to oppose arms sales to Israel and back a free and independent Palestine.
The party's founding conference in November is expected to finalise its permanent name, leadership structures and policies, with delegates chosen by lottery. Ahead of that, regional assemblies are planned to allow members to debate proposals and draft documents.