A further 20,000 local government workers are to be balloted on action in a row over pensions, amid fears of a wave of pre-election walk-outs.
Strike action could cause chaos in the run up to the election
Amicus said on Tuesday it will hold a strike ballot this month over plans to raise the retirement age.
Unison is already balloting members on action and the Transport & General Workers Union (T&G) is to hold a vote.
The Fire Brigades Union (FBU) is also expected to vote on similar action within a few weeks.
Construction union UCATT is also considering a strike ballot.
The unions are protesting at plans to raise their retirement age from 60 to 65 and change early retirement rules.
The changes to local government pensions are part of a wider review of all public sector pension schemes aimed at tackling rising pension costs.
Last week, the Employers' Organisation for local government spoke out against the strike threat, branding it "unjustified, precipitate and ultimately futile".
The group added that pension costs are rising as a result of longer life expectancy and those costs need to be controlled.
"We have a limited range of choices to control that cost," Rob Pinkham executive director of the Employers' Organisation said.
"Employees can pay a little more, can work a little longer or can receive a little less. Those are the only alternatives," he added.
Unison claims that the latest moves could mean that government staff face a cut of up to 30% in their pension if they retire at 60.
Wave of action?
T&G National Officer Peter Allenson added: "The government is intent on making hard-pressed council workers work longer for less."
UNIONS IN PENSIONS ROW
UNISON - currently balloting 800,000 members
T&G - 70,000 members to be balloted
AMICUS - 20,000 members to be balloted this month
FBU - 1,650 control staff expected to vote in a few weeks
UCATT - discussing a ballot of 25,000 members next week
"We reject a 'work till you drop' culture and the government should be setting an example in pension provision rather than pushing through cuts to benefits.
"Local government workers receive an average pension of just £3,800 a year, well below the average pension in the economy as a whole."
The T&G ballot will close on 8 March - the same day as Unison's ballot of 800,000 workers comes to an end.
Unison has previously warned that any action will take place soon after the poll closes, sparking fears of widespread industrial action in the run-up to the general election which is expected to take place on 5 May.