Northern Ireland's exams body has said a mistake was made in the marking of some A-level chemistry papers.
A flaw in the marking of the multiple choice section of the A2 chemistry paper was discovered on Friday.
It meant 151 students from 41 schools and colleges received lower grades than they should have.
The Council for the Curriculum, Examinations and Assessment (CCEA) said the correct marks have been applied and candidates awarded the correct grade.
UCAS, the body which manages university applications, has been informed.
The CCEA's interim chief executive, Gavin Boyd, said an internal investigation was under way to discover how the incorrect marks were awarded.
He said the mistake may have impacted on the original offer made to students by universities.
The majority of students affected applied to Queens University Belfast, Mr Boyd said, adding that the CCEA had been assured by QUB that offers would be honoured in relation to the candidates affected.
"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," he said.
Education Minister Caitriona Ruane said she would ensure the findings of an investigation would be made public.
"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."
A dedicated helpline has been set up to support schools and students affected by this issue. The number is 02890 261403.