A meeting of more than 200 Unison and GMB members unanimously voted for a ballot last week after rejecting a pay offer from Brighton and Hove council.
The authority says its "modest rise" for 90 per cent of assistants, would put them on higher salaries than their colleagues in neighbouring East and West Sussex. A council spokesman said a third of the assistants would get increases of between 5 and 30 per cent.
But the unions say that some of the assistants, earning an average of Pounds 9,000 a year, would receive just a few pence extra an hour, while one in 10 is likely to receive an effective pay cut after three years when salary protection ends. They are also unhappy because the deal means the end of a special schools allowance worth pound;1,044.
The unions did agree a four-rung career structure that would allow assistants on the highest grade to be paid as much as pound;21,033 after nine months of negotiations.
Alex Knutsen, Unison's Brighton branch secretary, said: "The behaviour of the council throughout these negotiations has been quite disgraceful. It is relying on moral blackmail that schools staff will not take action because of the job they do."
A council spokesman said the package being offered would add an extra Pounds 1.7m to the current pound;8.8m salary bill for teaching assistants.