“Let's be in no doubt about it, the exam results that we've got today are robust, they're good, they're dependable for employers,” he said.
The wide-ranging criticism of the results – with some students in tears, as predicted grades were lowered by use of a computer algorithm – had prompted speculation about a further government rethink.
But Mr Johnson denied the process would rebound on the 2020 cohort, pointing to figures showing they were snapping up the university places sought.
He said: “Already I think that there's a record number of candidates, of students, who are able to get their first-choice course at the university of their choice.
“Plus, there's a record number of students, of pupils, from disadvantaged backgrounds who now as a result of these grades, will be able to go to university.”
Anger grew as it was confirmed that almost 40 per cent of A-level grades awarded in England were lower than teachers' predictions, after the intervention of the regulator Ofqual.
Some 36 per cent were one grade lower, while 3 per cent were down two grades, after adjusted estimates replaced exams cancelled because of the coronavirus pandemic.
School and college leaders demanded a review and said all fees for appeals should be waived.
The Association of Colleges said there were some colleges where more than half of their school-assessment grades have been ‘moderated’ downwards.
Geoff Barton, leader of the ASCL head teachers' union, warned ministers “against simply digging in their heels and insisting all is well.”
One student in Somerset, Lilly Keeley Watts, told ITV News she had missed out on a place at Durham University because she had received only a C in biology.
“That computer doesn’t know who I am and that I exist. I don’t think it’s fair at all,” she protested.
And, asked if she had a message for Mr Williamson, she replied: “We are real people - you are not just speaking to numbers in a system. There is actually real people here that are opening these envelopes and it determines their future.”
But the prime minister urged critics to look at “the big picture” and the ability for students who felt hard done by to launch appeals.
“Where pupils are disappointed, where they feel that they could have done better, where they feel that there’s an injustice been done to them, there is the possibility of appeal and they can resit, they can take a resit this autumn as well,” he said.
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