James Cleverly has become the third education secretary to be appointed in just three days as swathes of government ministers resigned in protest over Boris Johnson's leadership.
Concerns were raised on Thursday after there was for a time only one remaining minister left in the Department for Education in the crucial run-up to GCSE and A-level results.
She had been appointed after former education secretary Nadhim Zahawi replaced Rishi Sunak as chancellor when he resigned on Tuesday. She accepted the role amid the fallout of Mr Johnson’s handling of the Chris Pincher scandal.
Ms Donelan's move on Thursday left Baroness Barran as the last-standing minister in the department.
However, shortly before the prime minister announced his resignation outside Number 10 on Thursday afternoon, Mr Cleverly was appointed education secretary.
Mr Cleverly was previously a minister of state (minister for Europe and North America) in the Foreign, Commonwealth & Development Office (FCDO).
In Ms Donelan's letter of resignation - prompted by the prime minister's handling of the scandal over former deputy chief whip Chris Pincher - the Chippenham MP said she was “very worried about the prospect of no ministers in the department as we approach results’ day”, and that the “impact on students is real”.
She added that nonetheless she had pleaded with the PM on Wednesday to do the “right thing” for the country and resign, and that both the country and the Conservative Party were “more important than any one person”.
“In life we must always do what we believe is right,” she said in her letter to Mr Johnson.
“Above all I am here to serve the British public.
“I see no way that you can continue in post, but without a formal mechanism to remove you it seems that the only way that this... is possible is for those of us who remain in Cabinet to force your hand,” she said.
Ms Donelan said Mr Johnson had put colleagues in an “impossible position” and that as someone who “values integrity above all else” she had no other choice but to leave post.
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Geoff Barton, general secretary of the Association of School and College Leaders, said: “Obviously, this cannot possibly go on, and we would expect that a workable government will be put in place following Boris Johnson’s announcement that he will step down in the autumn.”
“There is also a very serious problem with teacher shortages that is facing schools and colleges right now, and severe funding pressures exacerbated by rising energy costs. All this needs sorting out," he added.
Mr Barton said his members would be awaiting details, adding there were “some very important decisions to be made in education”, such as the teacher pay award for 2022/23.
“We have been expecting the recommendation of the pay body and the Secretary of State’s response before the end of the summer term,” he said.
“There is then exam results due out in August which will require clear communications from government because grading standards will be different from the past two years and indeed different from 2019 too".
Ms Donelan's letter came amid a flurry of further resignations on Thursday morning, before Number 10 sources confirmed the PM would resign as Tory party leader on Thursday following urgent calls for him to step down.
Caroline Johnson, vice-chair of the party and a member of the Commons’ education select committee, also left her post on Thursday morning, writing to Mr Johnson that the “cumulative effect of your errors of judgement and domestic actions have squandered the goodwill of our great party”.
On Thursday morning, Chancellor Mr Zahawi urged Mr Johnson to resign in the wake of the Pincher scandal which has caused multiple resignations across government.