The leaders of three of Britain's biggest unions are to meet to discuss plans to merge into a 'super union' with more than two million members.
Amicus believes the merger would create a "major force"
Leaders of Amicus and the Transport and General Workers' Union support the move and will be trying to persuade the GMB's executive to give its backing.
Supporters say a merger would boost negotiating power.
But some GMB members are thought to want to proceed more slowly to consider the implications.
BBC labour affairs correspondent Stephen Cape said the GMB also has its own problems with an investigation into alleged election irregularities.
Our correspondent said Amicus general secretary Derek Simpson and T&G leader Tony Woodley are "anxious to go ahead" with the merger.
Mr Simpson has said the merger would create a "major force" to protect working people's interests.
Mr Woodley said it was an "historic opportunity" for unions to grasp the industrial agenda.
Amicus has 1.2 million members and the TGWU has 800,000. Any merger would have to be approved by the unions' executives and their membership.
Supporters say the 'super union' would be big enough to stand up to global businesses and politically the block vote at the Labour Party conference would be worth 1.7m votes.
Along with the GMB and Unison, the TGWU and Amicus have worked closely together in the last year to hammer out a 56-point deal with Labour's leadership over equality at work, holidays and pensions - the Warwick Agreement.