More than 150 A-level students were awarded lower grades than they achieved, an exams body admitted today.
The grading errors occurred after a total of 935 chemistry papers sat in 41 schools and colleges in the North were marked incorrectly, the Council for Curriculum, Examination and Assessment (CCEA) said.
The CCEA said it was confident none of the 151 pupils who received the wrong grade would miss out on a university place as a result of the mistake.
The main exams body for the region has launched an internal investigation while Stormont's Department of Education has ordered an external probe.
The problem centred on the marking of the multiple choice section of the CCEA's A2 chemistry paper. Out of 1,024 entries for the exam, 935 required mark changes - 151 of which affected the final grade that was awarded.
CCEA's interim Chief Executive, Mr Gavin Boyd said: "Staff at CCEA are very disappointed by this failure and we apologise unreservedly for any distress this has caused to students, their families and teachers.
"On this occasion CCEA's quality assurance procedures did not ensure that the correct grade was issued for the candidates.
"This is unacceptable and it falls far beneath the standards we set ourselves as an organisation."
He added: "This issue may have impacted on the original offer made to students by universities.
"CCEA can confirm that the majority of students affected applied to Queen's University Belfast (QUB). CCEA has already been assured by QUB that offers will be honoured in relation to the candidates affected.
"As with QUB, we believe that all higher education institutions will honour the offers made to students and we are working with Ucas to ensure that this is the case."
Education Minister Caitríona Ruane said she regretted the episode.
"While I have been advised that no young person should miss out on their university place, this scenario should have never arisen," she said.
"This failure in the CCEA marking process has caused stress and inconvenience to the young people involved and their families, and I very much regret that.
"While I acknowledge the immediate and unequivocal apology from CCEA, the body has clearly fallen short of the high standards of accuracy the public and I, as minister, expect of a public examinations body."
A helpline has been set up to support schools and students affected by this issue. The number is 02890 261403.