A meeting of Unite’s National Industrial Sector Committee last week voted to reject the 2% offer from local government employers issued in December.
A consultative ballot will now be held with Unite members, although the result will be non-binding.
Rehana Azam, GMB national secretary, believed the offer was "the best we can achieve" but said the organisation was still to ballot members.
Unison will decide on its recommendation and the timetable for a consultation next week.
The offer covers more than 1 million council workers in England, Wales and Northern Ireland, and would lead to a 2% rises in 2018 and 2019.
The current consumer price index rate currently stands at 3.%, and the retail price index – which includes housing costs – at 4.1%.
A statement read: “The current pay offer will result in a further real terms pay cut for most of Unite’s local government members.”
Jim Kennedy, Unite national officer for local authorities, said: “Our members simply do not believe that the offer will result in enough members receiving a pay increase which is at least in line with inflation.
“Unite believes that if local government employers are prepared to enter into meaningful negotiations we can reach agreement to resolve our concerns as swiftly and amicably as possible.
“Local government has been decimated by the Conservative’s cuts and any pay settlement should be fully funded by central government.”
Azam said: "The local government pay offer is the best we can achieve through negotiations.
"GMB will now ballot every member so they have their say on pay."
