Momentum has faced a backlash for balloting is members on whether it should back Rebecca Long-Bailey for Labour leader - without offering an alternative.
The organisation's leadership voted unanimously to back Ms Long-Bailey and her flatmate Angela Rayner to become deputy leader.
The pro-Corbyn campaign group then sent an email asking its 40,000 members to vote "yes or no'' on whether to endorse the recommendations made by the elected National Coordinating Group (NCG) for leader and deputy.
Momentum said it would throw its weight behind Ms Long-Bailey and Ms Rayner should 50% of respondents to the survey agree.
But it faced a backlash for not giving members a choice of candidates.
Paul Mason, the former journalist and prominent Labour activist, who likened the move to a survey carried out by a "South American dictatorship".
He said: "As a Momentum member and somebody who has been protecting it against witch hunts, I think it is a very stupid thing to do.
"It's not for Momentum to decide who the Left's chosen candidate is - it's for the members. If someone other than Long-Bailey wins it is like saying we are going to become an opposition movement to the leadership."
Ms Long-Bailey has hit back at accusations she is the "continuity Corbyn candidate" saying the comparison "annoyed" her after years developing her own policies as part of Labour's top team.
But she has doubled down on Labour's "socialist agenda" telling Labour members in an article launching her candidacy in Tribune magazine: "I don’t just agree with the policies, I’ve spent the last four years writing them."
A Momentum source told the Mirror that Ms Long-Bailey was the only leadership candidate who shares Momentum's aims and values.
The email to Momentum supporters said: "The next few years will be crucial for our party, and it is essential we make sure Labour is led by a new generation of socialist MPs.
"The ballot consists of two recommendations, and those that receive a 'yes' vote from more than half of members will be adopted with immediate effect."
The potential backing of Momentum would be a huge boost providing campaigners and organisers to help Ms Long-Bailey's campaign.
It could also potentially gift her and Ms Rayner access to the organisation's vast database on Labour members.
While her competitors would not have access to official party data until later in the contest.
