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."