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."
More top news
Nearly half of all young people are bullied at school each day with many left feeling suicidal due to the experience, according to a survey
Thousands of pounds of NHS cash is being spent on holidays, aromatherapy and even summer houses for patients, according to research
Welfare reforms do not lead to more people being in work, a new study suggests.