More than 3,000 A-level and GCSE candidates were awarded the wrong marks last summer in the biggest exams blunder for nearly a decade.
As a result, 13 A-level candidates missed out on going to their first-choice university or course. A total of 3,350 candidates received wrong marks, 622 of whom were awarded lower grades than they should have been.
An inquiry into the error blamed a new online marking system introduced by the country's biggest exam body, the Assessment and Qualifications Alliance (AQA).
Isabel Nesbit, chief executive of Ofqual, the exams watchdog, said the failings by the AQA were "very disappointing".
Andrew Hall, chief executive of the AQA, said: "I deeply regret this failure and the impact it had on students, including the 13 who missed out on their first choice for course or university last summer. I want to reassure the public that all the affected students were notified and issued correct grades in October last year."
It is the largest remarking exercise since the introduction of a new A-level syllabus led to 90,000 A-level scripts being re-checked after it emerged markers were unclear of the new marking system. Twenty-four people switched courses or gained a new university place as a result of re-marking – and 170 candidates were awarded higher grades. After that incident, in 2002, the government set up an inquiry by former chief schools inspector Sir Mike Tomlinson.
Despite last summer's AQA error, none of the 13 students changed universities. The mistake was only spotted after the deadline for the university clearing and placement process had expired. Of the 13 who would have qualified for their first-choice university, four were accepted by their insurance choice university, seven were found a place through clearing or accepted on to another course and two were unplaced.
The errors emerged in a number of subjects, including history, law and classical civilisation at A-level, and religious education at GCSE. They were caused by a failure of the AQA's onscreen marking system to pick up all of the students' material for marking.
"Factors that contributed to the marking error included limited piloting of the new on-screen marking system, a lack of effective risk assessments and deficiencies in the role and training of examiners on the new system," Ms Nisbet added.
The AQA said it had introduced a more rigorous checking system and improved training for working on the system. Last summer was the first time it had made extensive use of its online marking system. In all, 3.3 million exam scripts were marked online.