Labour leader Ed Miliband will tell the TUC he is "absolutely determined" to change union membership in the party to an opt-in system.
Addressing TUC members on Tuesday, 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.
Mr Miliband's reform of trade unions would no longer see members automatically affiliated with Labour, as happens now.
Unions, though, fear the changes will relegate them to "placard carriers and cheque writers" and weaken or even sever the historic link with the party.
More top news
The poor response to the devastating fire at Grenfell Tower was "ultimately a failure of values", the Archbishop of Canterbury has said.
The president of the European Council, Donald Tusk, said he still hopes that the UK might change its mind over Brexit and decide to stay.
A blind veteran has launched a heartbreaking appeal for the return of his war medals, after they went missing while returning from Normandy.