Heads have said students missing out on exams this summer will be in "safe hands", as their grades will be decided by the "professionals who know them best".
Plans for schools to submit alternative exam grades based on classwork, mock tests and previous results are the "fairest approach to take in these exceptional circumstances", according to school leaders.
The NAHT school leaders' union has said there is "no perfect solution" to disruption caused by the coronavirus crisis, but welcomed Ofqual's plans, published for the first time today.
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The Association of School and College Leaders (ASCL) said the system will "allow students to progress to the next stage of their lives without hinderance".
Paul Whiteman, NAHT general secretary, said: "Many schools, colleges and their students will have been anxiously awaiting this information, and while there is not a perfect solution, this is pragmatic and the fairest approach to take in these exceptional circumstances.
"NAHT has provided the views of school leaders during the detailed discussions and processes of creating these new arrangements and we will of course continue to work with Ofqual in the days and weeks ahead.
"Of course, this is not a seamless solution. Students will have been expecting to go through a very different process.
"However, their grades will now be determined by the professionals who know them best; professionals who are well-equipped to make these judgements, and we hope that gives students confidence that they are in safe hands. Where pupils are not content, appeals are possible and autumn exams are being discussed."
He added: "We are pleased to see that there is no requirement for teachers to set additional tasks for students; schools will have enough knowledge and evidence prior to the school closures on 20 March to make their decisions.
"We also welcome that there is no requirement on schools and colleges to collate or provide any evidence to awarding organisations to support their judgements, a near impossible and needless task at this time."
Geoff Barton, ASCL general secretary, said: "We support Ofqual’s plan for the grading of A levels and GCSEs following the cancellation of exams, and call upon everybody to back this approach.
"We must all recognise this is a system forged in extremis. It is about making the best out of a difficult situation.
"We are confident that Ofqual’s plan is the fairest and most consistent way of grading these qualifications in these circumstances.
"Students can be assured their schools and colleges know them well, will assess their work with the utmost diligence, and that the process for standardising results will ensure a level playing field nationally.
"It will allow students to progress to the next stage of their lives without hinderance.
"Many of us have longer term concerns about aspects of the exam system as it currently stands. But that is a matter for another day. Change will require carefully considered reform.
"The immediate priority is fairness for this year’s A-level and GCSE students and we are all committed to making sure this happens."
Mr Whiteman added: "It will be extremely important for employers, universities and other places of work and study to play their part in supporting this year’s alternative arrangements, so that students can have confidence that their hard work will be judged correctly and valued in the same way as in previous and future years."