Despite his climbdown over alleged selection-rigging in his party's Falkirk candidate selection, Ed Miliband will persevere wih the union reforms which the row sparked.
On Tuesday, he will tell the TUC conference he is "absolutely determined" to press ahead, undeterred by warnings more unions will join GMB in cutting their party donations.
Mr Miliband wants members to opt in to joining Labour when they join a union, rather than being automatically affiliated.
Unions opposed to the reforms fear the changes will relegate them to "placard carriers and cheque writers" and the GMB announced it would cut its affiliation funds - money given to Labour by the unions, its biggest single source of funding - from £1.2 million to £150,000.
Mr Miliband will tell the conference: "We need to build a party truly rooted in the lives of all the working people of Britain once more.
"That is what my reforms are about. It is the right thing to do. We have to change.
"And I am absolutely determined to make this change happen."
The party was seeking a candidate for Falkirk, which former Labour - now independent - MP Eric Joyce will step down from in 2015 following a brawl at a Commons bar.
The row began with claims that the Unite union signed people up as party members without their knowledge to ensure its candidate, Karie Murphy, won. The union - Labour's biggest donor - strongly denied any wrongdoing from the start.
Both Murphy and local party chairman Stevie Deans were suspended, the claims were referred to the police, and the party carried out an internal inquiry. That inquiry cleared both Murphy and Deans of wrongdoing, and both have been reinstated.
Murphy has since withdrawn from the race, citing "reconciliation and unity" as her motives, though she said she was "shocked and saddened" by the whole episode.
Labour has stressed that its findings came after "key evidence" was withdrawn.
Police Scotland ruled in July that there were "insufficient grounds" for a criminal investigation. The case is also the subject of an investigation by the Information Commissioner's Office into whether data protection rules were breached.
The central party has kept in place restrictions on the constituency and will impose a shortlist of candidates for the 2015 general election.
Mr Miliband is apparently refusing to apologise to Murphy or Deans, despite calls from within the party for him to do so.
High-profile MP Tom Watson - who quit as the party's general election coordinator over the row - said: "Someone in the Labour Party owes Karie and Stevie Deans an apology.
Asked if it should come from Mr Miliband, he added: "It would be a very gracious act if he did."
But a senior Labour source insisted: "There is no prospect of an apology."
During the inquiry, Miliband got into a public row with Unite general secretary Len McCluskey, during which he proposed the recalibration of the party's historic union link.
Miliband also faces external pressure over the debacle, with Tories suggesting unions are "pulling the strings" of his party.
Conservative Party chairman Grant Shapps said: "If Ed Miliband can't even stand up to his union paymasters, how can he stand up for hardworking people?
"Weak Ed Miliband must now stop dithering, come clean and publish Labour's report into the Falkirk selection in full."