It shouldn't have happened and we take full responsibility

Gavin Boyd, CCEA Chief Executive

The exams body has apologised for the mistake and said an investigation was being carried out into how it happened.

CCEA Chief Executive Gavin Boyd said: "My staff are devastated but we realise that the people who will be most disappointed by this are the candidates themselves."

He said most pupils would only be marked down by one grade.

"The biggest movement will be out of the Grade A category," he said.

Gavin Boyd: Apology

However, some candidates will drop from an A to a C, with others dropping from a B to D grade.

Mr Boyd added: "I'm not going to put up any excuses. It shouldn't have happened and we take full responsibility.

"It hasn't happened in any other subjects."

The CCEA said the investigation would take in 24 members of staff.

Mr Boyd said: "I don't rule out any disciplinary action, but I am waiting for the results of the investigation."

Pupils should know their real result by lunchtime on Wednesday.

Almost 30,000 Northern Ireland students have received the results of their GCSE exams.

Once again girls have netted most of the top grades.

The CCEA said the results painted a picture of steady improvement.

Top grades

Overall, the number of pupils who got the top Grade A* has gone up to 7%, with girls claiming the lion's share.

Nine per cent of them got an A* grade, compared to 5.3% of boys.

Girls achieved 10% more than boys in the two top grades.

But this only represents about 70% of results, with the rest done through the English examining bodies.

The CCEA results are published two days ahead of the English boards.