Minister for Education Joe McHugh has said the State Examinations Commission told the Government it could not stand over the legality of Leaving Certificate examinations that were held in a different format to other years.
He told the Dáil that an original version of the exams was modelled, based on the public health advice made available to the Government. This would have involved just one exam taking place per day and the exams running from 29 July to September.
It would also have involved having papers that were one and a half hours long and staggered entry into exam halls.
"When we brought that to the partners around the table, we decided after getting advice from the State Examinations Commission that they couldn't stand over the legality and the validity a leaving cert that is not comparable to any other year that is what informed us."
"We in the department, me as minister, we made the decision based on the information that we had that it wouldn't be safe, it wouldn't be right and ultimately it wouldn't be fair," Mr McHugh said.
He was responding to questions from the Labour Party's Aodhán Ó Ríordáin who called for the publication of the advice made available to the minister on which he based his decision.
There was a tense exchange between the two over concerns raised by Mr O'Ríordáin about the impact of school profiling on pupils from disadvantaged backgrounds.
Mr McHugh said it was "an absolute disgrace" that Labour was "pedalling that line" because students "whether they are in a community school, a Deis school or a private school, they will not be discriminated against."
The minister told the house that while the Leaving Cert is important, it is life that matters.
The system that is in place now will allow students to progress to the next stage of their life, he said.
He added that there is no easy solution to the difficulties we face, and that this is the fairest and most equitable way to meet the challenges we face.
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Fianna Fáil has called for sanctions to deter parents from lobbying teachers who are grading Leaving Cert students.
Education spokesperson Thomas Byrne said there are legal vulnerabilities for the State and assurance is needed that no one will be "disadvantaged" or "advantaged" by the new system.
Deputy Byrne said he believes Minister McHugh is "not consulting with outside experts on adjusting grades" and there needs to be independent advice on the methodology being used by the Department of Education.
Sinn Féin's education spokesperson Donnchadh Ó Laoghaire said Leaving Cert students have endured "incredible anxiety".
He told the Dáil that while it was right to cancel the exams, he is not convinced that the right alternative was chosen.
Deputy Ó Laoghaire said there are plenty of students who know they can "turn the gas on" at the end of the year.
He asked how those students will get their grades and what guidance will be given to schools on material that can be used to devise a calculated grade.
Last week, the minister announced a newly-devised 'calculated grades' system to replace this year's Leaving Cert exams which have been cancelled this summer.
All students have been given the option of sitting written exams at a later date, as yet undetermined, should they choose.
Students will also be able to appeal the grades they are given.
Additional reporting Aisling Kenny & David Murphy