Minister for Education Norma Foley has said it is likely that around 6,500 Leaving Certificate students have been negatively affected by two errors discovered in this year's calculated grades process, however the exact number is not yet known.

Two coding mistakes in the system used to standardise this year’s marks mean an estimated 10% of students received a grade lower than they should have.

The full extent of the problem will only be confirmed after an independent review being carried out by a US company is completed.

Norma Foley said that students affected would be contacted "as quickly as we can" once that review is done.

However, students are facing days at least of uncertainty, as the minister could not say when that review would be completed.

Ms Foley said that no student would be disadvantaged by the mistake, which she said was contained in the code that had been provided by a Canadian company Polymetrika, which was contracted by the Department of Education.

Speaking at the Department of Education, she apologised to Leaving Cert students and said that no student would get a reduced grade in any subject as a result of this process.

The error relates to how Junior Certificate results were used in the moderation process carried out by the Department of Education.

Instead of picking up a student's two strongest subjects after English, Irish and Maths, the system captured their two weakest subjects. The system also mistakenly included Junior Cycle marks from the subject CSPE.

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The majority of the students who will be receiving an improved grade will receive an upgrade in one subject only, Ms Foley said, with a smaller number of students receiving an upgrade in more than one subject.

The minister said students who are entitled to a higher preference CAO offer will receive that offer and will be offered a place on that course this year "where at all possible".

The Department of Further and Higher Education has confirmed to RTÉ News that students who receive an improved CAO offer as a result of revised grades, but who have already begun and paid fees for another course, will not have to pay additional fees.

The minister said the department had carried out a series of further checks and had identified no further errors in the coding. 

When the revised grades are issued, the CAO and the Higher Education Institutions will establish which students receiving corrected results would have been eligible for a higher preference offer in previous rounds of the CAO process. 

Minister Foley said the department will work with the CAO and Higher Education Institutions to determine how those students who receive upgraded results can be facilitated to commence the course that they would otherwise have been offered in an earlier CAO round.

She said Leaving Cert exams will go ahead as planned in November and students will have the option of choosing to take the grade that benefits them the most.

Speaking on RTÉ's Six One News, Minister Foley said she was happy that she had taken the correct and appropriate steps.

She said there had been oversight of this process and the important thing was finding the error, acknowledging it and doing what needed to be done to rectify it for the benefit of the students involved.

The minister will make a statement in the Dáil tomorrow afternoon, with the opposition politicians making statements in reply.

She will return to the Dáil next week for a more detailed questions-and-answer session.

Harold Hislop, the Chief Inspector at the Department of Education, reiterated that no grades issued on 7 September will be lowered.

Also speaking at the Department of Education briefing, he said: "If you have a place in the CAO system, you hold that place.

"If you get a better offer in the CAO system, you have the option of taking that place."

Mr Hislop said almost 54,000 students made applications in the CAO process, and 24,500 of the places allocated were first preference offers.

"We know over 80% of people got one of their top three preferences," he said. "So clearly there are courses where they may well be satisfied with the course that they got."

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The Minister for Higher and Further Education, Simon Harris, has said an additional 1,000 college places may be needed to deal with the grades error.

He told the Fine Gael parliamentary party tonight that 3,000 successful appeals last year led to 600 places being needed.

Sinn Féin Leader Mary Lou McDonald described the situation as "a mess", while her party's education spokesperson Donnchadh Ó Laoghaire said the news was "absolutely extraordinary".

Mr Ó Laoghaire said there are so many questions now, including whether those students who may have been downgraded will be allowed access third level education this year or will they be deferred to next year.

Additional reporting Micheál Lehane, Paul Cunningham and Maggie Doyle