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.
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It will be a wet start tomorrow for many but things are looking up for the afternoon.