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."