Teachers in Wales and other parts of the UK are being asked to vote on industrial action after being offered below-inflation pay rises.
It could lead to strikes in the new year after unions rejected a 5% pay offer from the Welsh government.
Union boss Neil Butler said the offer was "not good enough" and amounts to a 5% decrease given inflation.
Education Minister Jeremy Miles said the unions' demands were reasonable but a bigger increase was unaffordable.
The two biggest teaching unions start sending ballots to members this week, with others due to follow.
