National school teachers have rejected the government's public service pay deal with a vast majority.
INTO members voted by 89% to turn down the draft agreement put forward last month.
It was rejected because it doesn't resole the issue of pay equality for new teachers.
The INTO Executive will consider the outcome of the ballot next month.
Deputy General Secretary for the INMO, Noel Ward, said equal pay remains the key issue and was the driving force behind such a decisive vote
"In staff rooms around the country, you have two teachers who are doing exactly the same work but are being paid on different rates based essentially on their age. The younger one is paid less," said Mr Ward.
He described the issue as "a festering sore".