Ed Miliband will stick by his controversial reforms of the party's links with unions, saying they will create a stronger Labour party, with a better chance of winning the next election.
In his speech to the TUC, Mr Miliband will acknowledge that some people are worried about change.
"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.
"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."
Speaking ahead of her appearance at the TUC conference, Deputy Labour leader Harriet Harman said at a local level the relationship between the party and the trade unions is "in practice, not working at all".
She added there is not much connection between the party and members at this level and "that is what Ed Miliband wants to change".
Shadow business secretary Chuka Umunna has rejected suggestions a cut in union funding would mean financial meltdown for Labour.
Speaking to the BBC Radio 4 Today programme, he said: "For us as parties, we've fundamentally got to change the way we operate if we are to make our parties relevant in the 21st century.
"Part of that involves making sure we've got a better connection with our communities and our people. I think the trade union link gives us a huge head-start over other parties in that respect, because it gives us a direct link to three million working people.
"What we are seeking to do is to ensure that we've got a better link to individual members."
Mr Umunna added: "Undoubtedly, we are going to take a hit in terms of our finances, but that doesn't mean that it isn't the right thing to do."
The Government has hit back at TUC claims that the UK remains in a job crisis, with a true scale of unemployment that far exceeds the official figures.
Once again the unions are showing just how out of touch they are.
Rather than welcoming the fact that employment is at a record high and the number of people claiming out of work benefits has fallen, they are determined to talk down both the economy and perhaps worse - the people who have actually worked hard to find themselves a job.
This is nothing but a cynical ploy from Labour's trade unions. The economy is growing and jobs are being created and they can't bear to welcome that.