Keir Starmer has been accused of risking “civil war” in the Labour party as he seeks to rewrite the rules that led to Jeremy Corbyn’s election as leader.

Starmer set out plans to switch to an electoral college system to decide future party leaders. He will argue the move will give greater sway to millions of trade union members – but the party’s left said it would also hand more power to MPs at the expense of ordinary members.

The move would be a return to Labour’s old system, under which MPs, party members and trade unions each had a third of the votes for a new leader. It was abolished by Ed Miliband in 2014, instead giving each member of the party and its affiliates one vote on any candidate on the ballot paper. Candidates must acquire the support of 10% of MPs, plus some support from constituency parties and trade unions to get on the ballot.

The change is set to be put to a vote at Labour’s conference in Brighton this weekend. The Guardian understands that Starmer has been buoyed by internal numbers that suggest he can command a majority of Labour members attending conference.

However, any vote is likely to be extremely tight. “This vote is not a given,” a senior Labour aide said. “It is a big risk for Keir and the chances of a big loss at conference is possible. But the calculation is it will look worse if he does nothing and looks like he is not in command.”

His decision to push through controversial party reforms risks the conference being dominated by factional rows. Callum Bell, the Momentum vice-chair, said: “Any attempt to take these rule changes to conference would mark the start of a civil war in the party. Grassroots members will have no choice but to mobilise all our strength to fight back against this bureaucratic attack. Conference will get very messy, very fast – and there is no saying who will come out on top.”

Allies of the Labour leader have urged him to make a definitive break with the Corbyn era and demonstrate his dominance in the party – though sources close to Starmer said he was not seeking conflict and instead wanted to offer unions a greater stake in the party.

Starmer will meet union leaders for a summit on Wednesday. He could still be forced into a U-turn if he does not win support from three key unions: Unison, Usdaw and the GMB. Unite and the CWU have already said they are opposed. Senior sources said Starmer would be forced to drop the proposals if two of the remaining unions opposed them.

A Labour source said Starmer would aim to convince the unions they would have more power under a return to the old system, pointing to the election of Miliband in 2010 when members and MPs had backed his brother, David, but the union vote had sealed the younger Miliband’s election.

A Labour source said trade union members could currently only vote in leadership elections if they had signed up as affiliate Labour supporters. The proposal would extend the vote to all trade union members who paid the political levy on their union fees – potentially expanding the leadership electorate to more than 2 million people.

Doubts have also been raised internally as to whether Labour’s deputy leader, Angela Rayner, would agree to the proposal but sources close to Rayner suggested that with union backing she would see it as a done deal.

Starmer outlined the proposals to his shadow cabinet on Tuesday, including a change to Labour’s policymaking process, which could deny members a chance to vote on the plans at party conference and avoid “an endless series of motions at party conference”.

He said: “Our rules as they are right now focus us inwards to spend too much time talking to and about ourselves and they weaken the link with our unions. I know that this is difficult – change always is – but I think these changes are vital for our party’s future.”

Unite’s new general secretary, Sharon Graham, said the plan was “deeply disappointing” and urged MPs to oppose it. “Unite believes in democracy and this move to reduce the entire membership to one-third of the vote while inflating the votes of MPs to one-third is unfair, undemocratic and a backwards step for our party.”