The head of one of England's biggest exam boards today apologised for marking errors which left hundreds of students with the wrong grades in their GCSEs and A-levels.
Andrew Hall, chief executive of the Assessment and Qualifications Alliance (AQA), admitted he was "concerned" that the problems could have affected students, particularly A-level candidates relying on grades for university places.
The exams regulator Ofqual today announced it has launched an inquiry into why around 600 GCSE, AS-level and A-level papers set by AQA were not fully marked.
Ofqual chief executive Isabel Nisbet said it is a "very serious matter".
"Ofqual is in full agreement with AQA that the first priority is to support students, schools and colleges," she said.
"AQA has assured us that they will make sure that the affected candidates are treated fairly and receive the marks their work merits as soon as possible. Once that is done, Ofqual's inquiry will focus on what went wrong and why it was not spotted sooner."
Around 150 A-level students are thought to have been given the wrong marks, along with 290 AS-level students and 190 GCSE students.
Mr Hall told the Press Association he is concerned that grade changes could have affected the students, particularly A-level candidates who rely on grades for university places.
"I don't know what that one grade change has done, I'm concerned about it, yes," he said. "We want to do everything we can to help these students."
Earlier, Mr Hall said: "We are extremely sorry for any distress caused as a result of the original incomplete mark and are reviewing our processes to ensure there is no repeat of this error.
"Errors of this type are extremely rare, but where they are identified it is our policy to ensure they are rectified and candidates are credited with the correct result."
AQA is the biggest exam board in England offering these qualifications.
The board discovered through its regular exam-checking procedures that the papers had not been fully marked.
In a statement, AQA said: "As a result of its ongoing exam-checking procedures AQA identified material in a small number of papers that had not been fully marked.
"This meant that some students received lower subject grades than they should have."
A number of students had marks changed but it did not affect their subject grades.
Mr Hall said that AQA's usual practice was to send individual questions to separate markers, rather than one marker marking a candidate's entire paper.
In this case, some papers did not have question numbers on them, and checks to ensure that no question fell through the net failed.
"I've launched an investigation to find out why that has happened," he said.
Mr Hall said the problem came to light after he and AQA's chief operating officer heard that someone had commented they had seen a script which appeared to have a question unmarked.
"What we have tried to do since we discovered this about a week ago ... is to get this sorted for the students."
AQA said it is working with examiners to mark the affected papers and award the correct mark or grade, and is contacting affected schools.
The deadline for schools and students to make inquiries about exam results has also been extended.
Anyone affected can call a dedicated helpline on 0844 2096614.Reuse content