More council workers are to start voting on whether to take strike action in protest at a below-inflation 1% pay offer.
Members of Unison are being balloted over the prospect of industrial action in England, Wales and Northern Ireland.
The union said the pay of some local government and school workers has fallen to the level of the 1990s after wage freezes and lower-than-inflation deals in recent years.
National officer Heather Wakefield said: "School and council workers are tired of the Government treating them like they are at the bottom of the pile and they are tired of going that extra mile for worse than nothing.
"In the wake of Government cuts and almost 500,000 job losses in councils alone, they continue to educate and support children in schools, maintain crucial local services, keep our communities clean and safe places to live, and protect the homeless and vulnerable."
Ballot papers will be sent to Unison's 600,000 local government members later this week.
The GMB has already announced a strike ballot among its 220,000 members and has warned of a walkout on July 10 if there is a yes vote.