Labour members will challenge Keir Starmer with a ballot demanding the UK voting system changes to proportional representation.
Members have selected the topic for debate at the party conference on Monday or Tuesday - despite the party leader appearing to rule out any change.
Talks on Sunday night produced a final motion that says Labour "must make a commitment to introduce Proportional Representation for general elections in the next manifesto".
During Keir Starmer's first term in No10, Labour would have to change the voting system to "a form of PR".
But it’s understood Sir Keir is “intensely relaxed” about the motion passing - as there is nothing forcing him to make it Labour policy.
One ally of the leader said those at the top of the party were not interested in or focused on changing the voting system - which would not happen before the 2024 election, even if Labour did change its policy now.
They added: “I’ll eat my hat if it makes it anywhere near our manifesto”.
Greater Manchester Mayor Andy Burnham urged Sir Keir to change his mind, saying he was “disappointed”.
It comes a year after unions blocked a Labour conference motion to change Britain’s voting system to proportional representation.
Local parties were 80% in favour of changing the voting system from first-past-the-post in 2021, but union affiliates voted 95% against.
Since then, Unison changed its position to back electoral reform. Campaigners now believe they have a “fighting chance” of it passing.
It comes after Keir Starmer said reforming the voting system was “not a priority for me”.
He told The Observer: “There are a lot of people in the Labour party who are pro-PR.
“But it’s not a priority and we go into the next election under the same system that we’ve got, first past the post.
“And I’m not doing any deals going into the election or coming out of the election.”
Greater Manchester mayor Andy Burnham urged him to reconsider.
He told Sky News: “I’m a little disappointed to hear the party saying it’s going to rule out electoral reform.
“I would say to them listen, hear the mood of conference on that particular issue.
"I hope conference will still vote in favour of that because we’ve got to maximise the chance here of a Labour-led government.”
Twelve motions were selected for debate by the members and unions in a “priorities ballot”, including Ukraine, health, social care, climate and violence against women and girls.
Members will also vote on workers’ pay, a year after they backed a £15 minimum wage but were rebuffed by the leadership.
Hilary Schan of left-wing pressure group Momentum said: “Following Kwarteng’s class war budget, now is the time for the whole labour movement to unite behind a bold alternative, based on fair pay, public ownership and democratic empowerment.
“These policies are overwhelmingly popular with the public and needed now more than ever during the cost of living crisis.”
Topics that did not pass the initial ballot included workers’ rights, drug reform, abortion rights, welfare and women’s state pension age.
