The ballot of GPMU members will take place as soon as the certification officer, acting for the Government, has given formal approval to the merger.
GPMU spokesman John Abbott said: "Any time now we're expecting him to give his formal approval and then we can get on with the ballot. It is close now."
Abbott stressed that the upcoming vote was not a repeat of January's poll, in which GPMU members voted to merge with Amicus rather than another trade union. "That was a consultative ballot, under our own rules, with no legal standing, but which confirmed our belief that Amicus was the best partner."