Moves to form a super union to create a "powerful" new force to represent public and private sector workers will be discussed today.
Delegates at the annual conference of the Public and Commercial Services union (PCS) will decide whether to press ahead with plans to merge with Unite.
The PCS has around 250,000 members mainly in the civil service, while Unite has more than 1.4 million members in private firms as well as the NHS, local government and elsewhere in the public sector.
PCS delegates, meeting in Brighton, will be asked to approve a special conference to debate the terms of any merger proposals.
Today's conference will consider a motion saying that a merger with Unite could create a new force for "fighting back" trade unionism, capable of bridging the public/private sector divide.
Exploratory talks took place last year and discussions are said to be at an early stage.
The PCS conference yesterday approved moves for a co-ordinated strike in July over public sector pay restraint, which could involve other unions, including Unite.