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.”
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.”
The Education Select Committee has said Ofqual representatives will appear in front of MPs on September 2.