Former Labour MP Zarah Sultana has accused Jeremy Corbyn and other members of a new left-wing party of “baseless attacks” on her character and said she is consulting lawyers.

It follows an email sent to supporters inviting them to sign up for membership of the new party at the cost of £5 a month or £55 a year.

Sultana had posted on social media encouraging people to join and claiming more than 20,000 people had done so.

But Corbyn posted a statement signed by four other independent MPs involved in the party in which he claimed the emails were “unauthorised” and said any direct debits set up should be “immediately cancelled”.

Allies of Corbyn have told the BBC they do not believe that anyone should be sued.

The former Labour leader declined to comment on Sultana’s latest intervention.

But speaking a the annual conference of the Peace and Justice Project, an organisation founded by Corbyn, he said he was “absolutely determined” the new party will be officially launched later this year.

He acknowledged that the process had been “quite difficult over the last few days” but denied that he and other independent MPs were controlling or directing the group, “despite what some people have discussed endlessly on social media”.

The row over the membership portal has revealed deep splits in the fledgling party, which was launched in July and is due to hold its founding conference in November.

Members will vote on its official name but it is currently using “Your Party” in campaign material.

On Thursday, Sultana described Corbyn and others as running a “sexist boys’ club” and claimed she had been sidelined by other members of the party’s working group.

She said the membership portal was “in line with the road map set out to members”.

The party said it had referred the matter to the UK’s data protection watchdog.

In a statement posted on X on Friday night, Sultana said that “a number of false and defamatory statements have been published about me concerning the launch of Your Party’s membership portal”.

The Coventry South MP said that they were “baseless attacks on my character are politically-motivated and I intend to hold to account those responsible for making them”.

“To that end, I have this evening instructed specialist defamation lawyers,” she added.

Sultana said that at “no point was members’ data misused or put at risk” and that “all funds received from members were ringfenced and protected in the appropriate manner”.

Three senior members of Your Party have issued a statement saying any donations made to the group “are safe” and suggested that people who want a refund will be eligible for one.

Former North of Tyne Mayor Jamie Driscoll, former Labour MP Beth Winter and the London-based former South African National Assembly member Andrew Feinstein, said they were “exasperated” and had “tried to mediate between Jeremy and Zarah for some weeks now”.

They added: “In blunt terms, we demanded a meeting to get it sorted. One side agreed, the other has not responded despite multiple attempts.”

They said a company, which is called MOU Operations, had been set up in their name to be the “custodian of the donations”.

According to documents on Companies House, MOU Operations was set up in April this year, more than two months before Corbyn announced he was launching a new party.

Driscoll, Winter and Feinstein said they would “speak to Jeremy and Zarah and make sure the mechanism for any refunds is shared with you”.

They added: “All the donations encouraged by both Zarah and Jeremy, or anyone else, went into this bank account, with their approval.

“The recent email launching a membership system uses exactly the same payment system, and goes into the same bank account.”