A GPMU spokesman said moves were progressing, and were on track for the proposed timetable.
Once formal documents have been approved, another ballot for a final endorsement would be called.
The spokesman said that this would be held by "mid-summer" at the earliest.
A postal ballot held in February endorsed the recommendations made by the GPMU's executive council to merge with Amicus.
Of the GPMU's 100,000 members, 26,976 endorsed merger talks with Amicus, while 9,803 cast their vote in favour of the blue collar Transport & General Workers Union.
The decision was supported by 25 of the GPMU's 27 branches.
GPMU merger at legal stage
The GPMUs executive council is due to consider formal legal documents within the next month, as its merger with manufacturing union Amicus progresses (PrintWeek, 12 February).
