More than a quarter of Labour members are set to drop Sir Keir Starmer and vote for Jeremy Corbyn's "Your Party" at the next General Election.
Pollsters at Survation have revealed that 28 per cent of signed-up party backers would switch allegiance to their former leader, while an overwhelming 59 per cent have urged Mr Starmer to veer to the left.
And in a rebuke to the Prime Minister's attempts to court the right-wing on migration, two per cent of Labour members said he should move in that direction.
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Despite the warning over a quarter of party members turning to Mr Corbyn's hard-left offering, 51 per cent said they feared he would split the left-wing vote and send either the Conservatives or Reform UK into Government.
The disgraced ex-Labour leader himself, responding to the data, said: "For too long, people have been denied a real political choice.
"Not any more: 700,000 people have already signed up to build a real alternative to inequality, poverty and war.
"One based on public ownership, wealth redistribution, housing justice and peace. This is just the beginning. We are an unstoppable movement, and we are never going away."
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Jeremy Corbyn and Zarah Sultana have been the architects of Your Party
| PAA Labour source, however, defended the party's performance and said the Prime Minister was focused on creating "a fairer Britain for working people".
The data was collected by Survation on behalf of LabourList, and surveyed 1,021 Labour members this month.
A location and date for the launch conference to formally launch Your Party and hold leadership elections have not been confirmed.
The founding process is being orchestrated by Mr Corbyn, Zarah Sultana and four independent MPs on a pro-Gaza platform.
Diane Abbott said on Thursday that she advised Mr Corbyn not to launch a new political party as she felt it would struggle to succeed under the first-past-the-post system.
She told the Edinburgh International Book Festival: "There were people around Jeremy encouraging him to set up a new party, and I told him not to.
"It's very difficult under the first-past-the-post system for a new party to absolutely win.
"If it wasn't first past the post, then you can see how a new party could come through, but I understand why he did it."
Some 28 per cent of Labour MPs are considering supporting Mr Corbyn
| PAMiss Abbott said last month she would not be joining Mr Corbyn's party, despite the pair being long-time friends.
She described Ms Sultana as a lovely person who is full of energy.
Miss Abbott said that the party could get votes from people who were not necessarily left-wing but were underwhelmed by Labour's actions over the past year.
The Labour Party has suffered a major blow with its membership numbers, as almost 200,000 have cancelled their memberships in the past five years.