Ofqual chief regulator Sally Collier has quit in the wake of the A-level and GCSE results U-turn in England.
She will be replaced by her predecessor Dame Glenys Stacey, the exams regulator said.
In a statement on Tuesday Ofqual said Ms Collier had decided “that the next stage of the awarding process would be better overseen by new leadership”.
It follows a Government U-turn away from awarding students in England grades based on an algorithm which appeared to boost private schools’ performance, and instead to award pupils their teacher assessed grades.
The controversial algorithm had led to many students having their results downgraded earlier this month.
Ofqual said: “The Ofqual Board supports Sally in this decision, and thanks her for her leadership and service over the past four years, which has included overseeing the successful introduction of an entirely new set of GCSEs and A-levels, and a new grading system.”
Education Secretary Gavin Williamson, who has also faced calls to resign, said: “Following Sally Collier’s decision to step down as Ofqual’s chief regulator, I’d like to thank her for the commitment she has shown to the role over the last four years and wish her well for the future.
“I welcome Ofqual’s announcement that Dame Glenys Stacey is to assume a temporary leadership role as acting chief regulator and also the new internal governance arrangements put in place with Ofsted support.
“This will make sure Ofqual can fully focus on the important functions it must deliver as the independent regulator for qualifications, examinations and assessments in England.”
He said his department will “continue to work closely with Ofqual’s leadership to deliver fair results and exams for young people”.
Dame Glenys, who was chief regulator between 2011 and 2016, has been asked to take on a temporary leadership role as acting chief regulator until December.
Ofqual said: “She will be supported by a new committee of the Ofqual Board, which will include one or more of the current Ofsted Board members.
“This new committee will be chaired by Amanda Spielman and will oversee the work of Ofqual to the end of the year.
“Roger Taylor remains Ofqual chair.”
It was also announced that Ofsted will provide additional staff to support Ofqual during the autumn, if needed.
Ofqual added: “Taken together these arrangements will ensure that Ofqual has the extra capacity, support and oversight it needs both to tackle the remaining issues from this year’s awarding process and to ensure that next year’s arrangements command public confidence.”
The Education Select Committee has said Ofqual representatives will appear in front of MPs on September 2, while Mr Williamson is due before the committee on September 16.
Earlier on Tuesday Boris Johnson admitted that in hindsight the Government “might have done some things differently” regarding results.
The Prime Minister said: “Yes, you know if we had to do it again, we might have done some things differently, I’m certainly not going to deny that.”
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), described Ms Collier as a “highly intelligent, principled, and thoughtful person” and said ministers have questions to answer around what went wrong with results.
He said: “This move (Ms Collier stepping down) follows the failure of the statistical model that led to this year’s grading fiasco, but the fault is not hers alone.
“Ministers have questions to answer over the extent to which they scrutinised and challenged the methodology and reliability of the statistical model, particularly given the enormity of the task and the importance of getting it right.
“Schools and colleges put their trust in the Government over this matter, dutifully followed the processes advised by Ofqual, and now feel badly let down by the ensuing debacle. They deserve some answers and so do their students.”
The ASCL has written to Mr Williamson to request an “immediate independent review to rapidly establish what happened and what went wrong”.
Mr Barton added: “We believe that public confidence has been so badly damaged that full transparency is essential.”
Liberal Democrat education spokesperson Layla Moran accused Mr Williamson of not taking responsibility “for the combination of initial school return failure and the more recent exam fiasco” and claimed he was letting someone else “take the blame”.
Reiterating a call for Mr Williamson to resign, she said: “He has lost the trust of students, parents and teachers who he failed to consult, leaving universities and colleges in chaos.”