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By Iain Watson
Political correspondent, BBC News
Keir Starmer will re-open a bitter internal dispute just days ahead of his party's conference.
In a risky move, he'll try to convince trade unions to back a plan to scrap the leadership rules which enabled Jeremy Corbyn to get elected.
Currently grassroots members elect the leader - a system introduced by Ed Miliband.
But Keir Starmer wants to return to an 'electoral college' which would mean rank and file members would only account for a third of the votes in a future leadership contest.
MPs would be given a further third of the vote - a move backed by 'moderate' members of the party as they believe elected politicians more closely reflect the views of constituents and not just activists.
The unions would also be given a third of the vote.
That may, on the surface, seem to appeal to the unions, but Labour politics is never that simple.
'DeCorbynisation'
Between them, the unions account for half the votes at Labour conference, so they will have huge influence over whether the leadership rules will change.
And for those under left wing leadership, they see the proposed rule changes as an attempt to 'deCorbynise' the party - reintroducing rules which gave MPs too much say at the expense of the members.
In her first broadcast interview, Sharon Graham - the new leader of the influential Unite union - condemned the move as "an error" by Keir Starmer - and has told him so.
She said he hadn't floated the idea with her in advance.
And the left wing group Momentum warned Sir Keir that he was risking "civil war" in the party.
'Huge error'
The Unite leader told the BBC: "The trade union movement will be telling him to think again.
"People will remember that at their conference, Labour talked about rules not issues.
"That's a huge error for them.
"We are almost trying to save them (the Labour leadership) from themselves.
"This is not the path to go down."
And she has written to Labour MPs urging them to oppose Sir Keir's position.
She argued: "This proposal to reduce the membership to one third of the vote, while inflating the vote of Labour MPs is unfair, undemocratic and a backwards step."
High wire
On Wednesday, Keir Starmer will argue his case directly to the unions which affiliate to the Labour Party.
Insiders tell me that very little preparation has been done, and that the Labour leader has no guaranteed support.
For his plan to succeed he would need to win over three big unions - the GMB, shopworkers union Usdaw and public service union Unison.
If it is clear that too many unions are opposed, the proposal won't go to Labour's conference.
If, however, two of those unions back him and another agrees to abstain, the proposal would go to Labour's National Executive Committee on Friday and then on to the full conference at the weekend.
Keir Starmer has been seen as cautious, but this would be a high wire act as he could be defeated either before conference or on the conference floor.
But allies are praising him for leading from the front and hope his authority will be strengthened if he wins.
I am told that he is willing to negotiate on the precise make up of the electoral college - and may be willing to give unions a bigger say in a leadership contest in order to get the proposal through.
He will also propose to make it more difficult for local parties to oust - or deselect - MPs.
The left wing group Momentum condemned the moves - its vice chair Callum Bell said: "These rule changes would mark the start of a civil war in the party. Starmer holds the membership in contempt."