He will face tough questioning from delegates at the TUC Congress in Bournemouth after delivering a crucial speech.
He is expected to be asked about his plans for union members to opt in to Labour affiliation rather than being automatically affiliated, a move which has already led to the GMB cutting its funding of the party by more than £1 million.
Len McCluskey, general secretary of Unite, has urged Labour's leadership not to be "embarrassed" by its historic link with unions, amid warnings that it will be weakened by the reforms.
He told a fringe meeting at the TUC: "They should embrace it, because we are the real voice of decent working people. Our Labour leadership has to start demonstrating they are on the side of ordinary working people, with policies that take us away from the path to poverty."
Miliband will hold private talks with senior union leaders today, following weeks of in-fighting between the two wings of the labour movement.
Union officials have called for an end to the row which flared after Unite was accused of trying to influence the selection of a Labour candidate in Falkirk.
The union was cleared after an internal review, but the repercussions have continued after Miliband said he wanted to review the relationship between Labour and unions.
A special conference will be held next March to approve the reforms, but action has already been taken which will hit Labour's finances.
The GMB said it will reduce the number of its members it affiliates to Labour, leading to a reduction of around £1.1 million from January, while Unison said its money will also reduce from next year.
In his speech to the TUC, Miliband will acknowledge that the reforms he proposes are a massive challenge and that some people are worried about change.
But he will stress that the current system of affiliation for trade unionists "must change" so that they have a real choice as individuals about being part of the party.
"I am a One Nation politician, and One Nation is about governing for the whole country. To do this, we are going have to build a new kind of Labour Party. A new relationship with individual trade union members.
"Some people ask: what's wrong with the current system? Let me tell them: we have three million working men and women affiliated to our party. But the vast majority play no role in our party. They are affiliated in name only. That wasn't the vision of the founders of our party. I don't think it's your vision either, and it's certainly not my vision.
"That's why I want to make each and every affiliated trade union member a real part of their local party, making a real choice to be a part of our party so they can have a real voice in it.
"This is an historic opportunity to begin bringing people back into the decisions which affect their lives.
"It means we could become a Labour Party not of 200,000 people, but 500,000, or many more. A party rooted in every kind of workplace in the country, a party rooted in every community in the country, a genuine living, breathing movement.
"Of course, it is a massive challenge. It will be a massive challenge for the Labour Party to reach out to your members in a way that we have not done for many years and persuade them to be part of what we do, and, like anything, that is hard, it is a risk. But the bigger risk is just saying let's do it as we have always done it.
"It is you who have been telling me year after year about a politics that is detached from the lives of working people. We need to build a party truly rooted in the lives of all the working people of Britain once more.
"That's why we must have the courage to change. I respect those who worry about change. I understand. But I disagree. It is the right thing to do."
Miliband accused the Prime Minister of "writing off" unions and their members.
"How dare he insult people, members of trade unions, as he does? How dare he write off whole sections of our society?
"We know from recent experience what happens to political leaders who write off whole sections of a country. That's what Mitt Romney did when he talked about the 47% of people who would never vote for him. And look what happened to him. My job is to make sure that's what happens to David Cameron as well."