Shadow Treasury secretary Rachel Reeves said she was "confident" more trade union members would sign up to the party when automatic affiliation is stopped.
"Of course we welcome the support that we get from the trade unions but this is a decision for the GMB," she told BBC Radio 4's Today.
"Since the last general election, membership in my constituency party in Leeds West has increased by 50 per cent. We've got more members, more trade union activists getting involved in the party because they like what Ed Miliband has to say.
"So I am confident that more people will sign up, get involved in the Labour Party, and come out campaigning. But that openness and transparency is really important and that is what Ed is trying to do."
More top news
It will be hotting up as we approach the weekend as we draw in very warm air from the Continent - it'll feel more like summer than spring.
A new scam aimed at people who own Apple iPhones steals their account information through a text.
Tomorrow’s Congress is expected to be dominated by claims of North Korea’s nuclear prowess.