Hundreds of activists have voted to create a "super-union" by merging Amicus and the Transport and General Workers' Union (T&G).
The merged union would have around two million members
Tony Woodley, T&G general secretary, said the merger was about "working class unity".
The decision follows a meeting of more than 300 T&G members and both trade unions' members will be balloted on the merger in February.
Together, the two would create the UK's biggest union of some 2 million people.
Amicus has 1.2 million members and the T&G has 800,000.
T&G members debated how a merger would affect links with the Labour Party and TUC, industrial relations and the organisation of the new body.
If a merger move does win backing at the conference, the "super-union" will be launched in May 2007.
Amicus has already given its backing to the plan.