Hundreds of students have been given the wrong grades for GCSE and A-level papers they took this summer, the exams watchdog warned today.
Ofqual has launched an inquiry into one of the country's biggest exam boards – the Assessments and Qualifications Alliance (AQA).
Isabel Nisbet, the watchdog's chief executive, said an estimated 600 papers had not been marked properly by the board's markers and added that the matter was "very serious".
Some 150 A-level students are thought to have been given lower marks than they deserved, as well as 290 AS-level and 190 GCSE students.
AQA informed Ofqual and is now contacting all the schools thought to be affected. It has extended the time in which teachers can query a pupil's exam result.
Andrew Hall, AQA's chief executive, said the board was "extremely sorry for any distress caused". He said he was reviewing marking procedures to ensure there was no repeat of the 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," he said.
"We want to ensure we support schools and individual students as we communicate the revised marks and grades and answer any questions or concerns they have."
Nisbet said Ofqual would focus on what went wrong and "why it was not spotted sooner".
In June, 584 students joined a Facebook campaign against AQA's English literature A-level exam.
The teenagers said the literature extracts they had to read were too long to be properly analysed in the time they had.
Bianca Khatri, a student who sat the exam, said: "AQA decided to give us 4 extracts that were each at least a page long - students ultimately had 30 minutes reading time over the whole paper, which meant students realistically had 7-8 minutes per extract to analyse each text and look for language devices".
"This meant with the inadequate reading time, students had to leave out crucial information from each extract in order to complete the exam on time," she said.
In January, more than 3,000 students sent furious messages to a Facebook group about an AQA biology A-level exam.
At the time, AQA, said it would take students' worries into account when marking and awarding grades in the subject.
It said it was the first time a new curriculum for the exam had been studied and this could have caused uncertainty over what would be tested.
Pupils described the exam as a "disgrace" and feared it could jeopardise their chances of a university place. The exam bore no relevance to specimen papers, they said.
Hundreds posted messages labelling the paper "awful" and "ridiculous". --Jessica Shepherd ,4 October 2010 12.58 BST