Two senior officials have stood down in the wake of the A-level results fiasco, which Boris Johnson has since sought to blame on a “mutant algorithm”.

The Department for Education (DfE) announced on Wednesday its permanent secretary Jonathan Slater was standing down, while Sally Collier resigned from her role as head of exams regulator Ofqual a day earlier.

It comes after the Government was forced to make a U-turn over the way A-level and GCSE results in England were graded, from a controversial algorithm devised by Ofqual to teachers’ predicted grades.

Here is a timeline leading up to the latest developments:

– Wednesday March 18

Prime Minister Boris Johnson announces that schools and colleges in England will close from March 20 to slow the spread of Covid-19 in the UK.

A-level and GCSE exams are cancelled but the Prime Minister says there are plans for students to receive their qualifications.

– Friday March 20

Education Secretary Gavin Williamson announces that students will be given grades based on teacher assessments for each subject, which will then be submitted to the exam boards.

The Department for Education claims the calculated grades will be “indistinguishable from those provided in other years”.

– Friday April 3

Ofqual instructs teachers to provide grades that reflect “fair, objective and carefully considered” judgments of the results they believe each student would have been most likely to achieve.

If the grading judgments appear to be more severe or generous than others, exam boards will adjust them, the regulator says.

– Friday July 10

The Commons Education Select Committee warns that pupils could miss out on the results they deserve as the appeal process risks being “unfair” for disadvantaged students.

– Tuesday July 21

Qfqual says GCSE and A-level students’ results are likely to be higher than in previous years.

The regulator says that if the teacher-assessed grades had not been adjusted through the standardisation process, this year’s results would have been 12 percentage points better than in 2019 at A-level and nine points at GCSE.

– Tuesday August 4

Some 124,564 exam results for school pupils in Scotland are downgraded by the Scottish Qualifications Authority (SQA) – 26.2% of the total moderated.

– Tuesday August 11

Scotland’s Education Secretary John Swinney announces that school exam results downgraded by the moderation process will revert to the grades estimated by pupils’ teachers.

Hours later, Mr Williamson says A-level students in England will be able to use grades from mock exams to appeal against their results.

– Wednesday August 12

Mr Williamson tells the BBC the system will deliver “credible, strong results” for the overwhelming majority of young people, describing it as “fair and robust”.

– Thursday August 13

A-level pupils receive their results, with thousands finding their grades have been downgraded by exam boards.

According to Ofqual, 39.1% of pupils’ grades in England were marked down by one grade or more – amounting to 280,000 entries being adjusted.

– Saturday August 15

Mr Williamson tells The Times there will be no U-turn on the grading system, claiming that moving to the Scottish model would lead to “rampant grade inflation”.

– Sunday August 16

Hundreds of A-level students protest against the results in central London, while education unions criticise the Government for its handling of the issue.

– Monday August 17

The Government makes a U-turn over the way A-level and GCSE exam results are awarded in England following mounting pressure on Mr Williamson and Mr Johnson.

Mr Williamson apologises for the way the process was handled.

– Wednesday August 19

Schools and colleges are told not to issue BTec results to students on Thursday after another last-minute U-turn on grading.

Exam board Pearson said it would recalculate results based on centre-assessed grading and provide them the following week.

– Thursday August 20

GCSE students receive their results, while regraded A-level marks are also released.

The proportion of GCSE entries receiving the top grades – at least a 7 or an A – is a record high at 25.9% in England.

Thousands of A-level entries are also upgraded.

– Tuesday August 25

Btec students begin receiving their grades, while Mr Johnson admitted that in hindsight the Government “might have done some things differently” in regard to the exams fiasco.

Ofqual announces its chief regulator Sally Collier has quit her role, as she decided “that the next stage of the awarding process would be better overseen by new leadership”.

– Wednesday August 26The DfE announces that its permanent secretary Jonathan Slater will stand down because “the Prime Minister has concluded that there is a need for fresh official leadership”.

Meanwhile, Mr Johnson tells pupils at a school in Leicestershire that grades were “almost derailed by a mutant algorithm”.

Shadow education secretary Kate Green accused the Prime Minister of “shamelessly trying to avoid taking responsibility for the exams fiasco that his Government created”.