The Leaving Cert was thrown into further disarray after the Association of Secondary Teachers' Ireland (ASTI) rejected the legal protection being offered to teachers to engage with the calculated grades process as falling "far short of what is required".
The union dropped the bombshell after the Attorney General and Cabinet signed off on an indemnity for teachers and schools in the event that a student challenges their grade.
It creates more uncertainty for the 61,000 Leaving Cert candidates who have already been through months of turmoil.
The indemnity emerged after weeks of discussions on the calculated grades process - which is replacing the summer Leaving Cert exams - leading to the publication of guidelines for teachers yesterday.
While the other second-level teachers' union, the TUI, welcomed the "clear legal indemnity" and the safeguards it offers, the ASTI said its legal advice was that it could potentially lead to personal liabilities for costs for second level teachers.
"This position is unacceptable," the union said.
The ASTI said it would continue to engage with the Department of Education, but, in the meantime, was advising its members "not to undertake any work on the process".
A Department of Education spokesperson said further clarification had been provided to the unions and engagement in relation to that was ongoing.
The publication of the guidelines was intended to clear the way for teachers to take the first steps in a complicated process to arrive at calculated grades for each student in each subject.
But the ASTI position has raised doubt as to when its members, who work in more than half the second-level schools in Ireland, will engage.
Earlier, the Cabinet approved the indemnity to apply where an individual teacher, principal or board of management/Education Training Board is sued.
It would be conditional on their co-operation with the State in defending any legal cases that may arise if students are unhappy with their grades.
In order the get the protection, the person/board would have to show that they acted in good faith by making "every reasonable effort to carry out their role in accordance with the guidance provided".
Calculated grades will be prepared for every student and they will be invited to opt in to receive them.
If they choose not to, they can sit the traditional exams, on a date yet to be determined, but unlikely to be earlier than November.
A student who opts for calculated grades and is not happy with the outcome may also sit the traditional Leaving Cert, in one or more subjects, when it takes place.
The guidance deals with a wide range of issues, including how teachers /principals should approach the marking and the consequences for anyone trying to influence the outcome.
The first step in the process involves subject teachers awarding estimated marks and a class ranking for each student.
Following in-school reviews the principal will send the markings/rankings to the Department of Education for national standardisation, and it will issue calculated grades.
Standardisation will involve a comparison of how students in a school have fared at Leaving Cert over the past three years and will also review the performance of this year's candidates against their overall performance at Junior Cycle.
Any parent, student or other person who persists in seeking to influence the marking will be reported to the Department of Education.