The TUC is to call a summit this autumn to discuss plans by three of the UK's largest trade unions to merge into a 'super-union'.
Members of the Transport & General Workers' Union, Amicus and the GMB are to be balloted on the proposed merger, which is due to launch in January 2007.
The super-union would represent 2.5 million workers - 40% of the membership of the TUC, and there are fears that the TUC and smaller unions will be marginalised.
Delegates at the TUC Congress on Thursday backed a motion calling for a working party to be set up to discuss the issues.
TUC general secretary Brendan Barber told delegates there would be "no pre-determined outcomes" but said he hoped the meeting would result in firm proposals being brought to next year's Congress.