The Association of Secondary Teachers Ireland (ASTI) has withdrawn from discussions on this year's Leaving Certificate.
The union said Government proposals are not viable.
General Secretary Kieran Christie says it is unacceptable a plan is being developed which will effectively see students preparing for two versions of a Leaving Cert.
He says that would involve calculated grades being the dominant option, and exams filling in assessment gaps.
The union is calling on Minister for Education Norma Foley to re-establish the focus of the talks so that a meaningful Leaving Certificate experience is provided to students.
“We entered a process in good faith to explore the position whereby if the Leaving Certificate or elements of it do not go ahead, a fair and credible choice or option would be available to students,” said Mr Christie.
“It is clear to us that the approach being developed would not provide the meaningful Leaving Certificate experience this cohort of students deserves.”
'Taken aback'
The Department of Education said it is “disappointed” and “taken aback” by the ASTI's decision to pull out of negotiations.
A spokesperson said: “At no stage of discussions today did the ASTI indicate its intention to take this action. The Minister and her officials will continue to engage with all education representative bodies, including the teacher unions, to provide Leaving Certificate examinations and a corresponding measure for examination students.
“Fairness and certainty for the class of 2021 is at the centre of what we want to achieve in these engagements. We are happy to engage with all concerned to achieve this objective.”
Special education
Meanwhile, thousands of children with additional needs returned to special schools across the country today.
4,000 students returned to in-person learning for the first time since before Christmas.
50 per cent of pupils are returning on different days.
Special classes in primary and secondary schools will resume later this month.
