Government ministers have been told they still have questions to answer over this summer’s grading chaos, following the resignation of the Ofqual chief regulator in the wake of the A-level and GCSE results U-turn in England.

Sally Collier 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”.

The Ofqual Board supports Sally in this decision, and thanks her for her leadership and service over the past four years

It follows a Government U-turn away from awarding students in England grades based on an algorithm and instead pupils have been awarded their teacher assessed grades.

The controversial algorithm had appeared to boost private schools’ performance and led to many other students having their results downgraded earlier this month.

But following Ms Collier’s resignation, the Association of School and College Leaders (ASCL) said questions remain for ministers over what went wrong, while the National Education Union (NEU) said issues “run far deeper than the actions of one chief executive”.

Liberal Democrat education spokeswoman Layla Moran accused Education Secretary Gavin Williamson of letting someone else “take the blame” for what had happened, and reiterated a call for him to resign.

The regulator 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.”

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.”

Mr Williamson thanked Ms Collier for her “commitment” to a role she held for four years and welcomed Dame Glenys’ appointment and the new internal governance arrangements.

He said: “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”.

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.

Committee chairman Robert Halfon said Ms Collier had agreed to appear in front of MPs before her resignation and he hoped that she still would.

Geoff Barton, general secretary of the ASCL said Ms Collier’s resignation “follows the failure of the statistical model that led to this year’s grading fiasco, but the fault is not hers alone”.

He added: “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.”

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.”

NEU joint general secretary Kevin Courtney said: “Sally Collier had no option but to follow through Michael Gove’s reforms of GCSEs and A-levels, which set up many of the problems of 2020.

“We have no sure way of knowing where the balance of fault lies, but we can be quite certain that Gavin Williamson gave direction to Ofqual that there should not be grade inflation and all candidates should get a fair grade.

“He must have known that both those directions are incompatible.”

Mr Courtney also criticised Ms Spielman’s involvement on the new committee, saying: “Ofqual already had an image problem amongst teachers and heads, but the parachuting in of Amanda Spielman from Ofsted makes an absolute mockery of Ofqual’s purported independence.”

Paul Whiteman, general secretary of school leaders’ union NAHT, said public confidence in the process for exams and assessments in the next academic year “needs to be re-established quickly”.

He said: “The real risk that there will be further disruption next academic year must be recognised and contingency plans for a range of possibilities need to be put in place.

“Students should not have to endure the uncertainty and chaos experienced this year.”

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.”