Asking teachers to assign grades that pupils "might have been expected to achieve" in next year's exams would not "make up for that lost learning", the Ofqual chief regulator has said.
And moderating teacher assessment fairly is "difficult to do in a way that commands the confidence of parents and students", according to Dame Glenys Stacey.
In an article outlining how Ofqual will set standards for "fair" exams and assessments in 2021, Dame Glenys acknowledged that "some will ask why teacher assessment is not figuring more prominently" in the plans.
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She argued that, if teachers were to suggest grades based on exam projections, this would not restore learning lost during the pandemic.
"If teachers were to assess students and suggest a grade that students might have been expected to achieve in an exam, it still would not make up for that lost learning," she said.
"And, as the summer of 2020 showed us, moderating teacher assessments fairly is difficult to do in a way that commands the confidence of parents and students."
The news comes on the day that the education secretary announced students taking GCSE and A-level exams next year will be awarded more generous grades in line with those awarded in 2020 to compensate for disruption to their schooling caused by the pandemic.
Gavin Williamson said students in England will receive advance notice of some topics ahead of tests – as well as exam aids when sitting papers, such as formula sheets or vocabulary lists – to ensure that this cohort of students is not disadvantaged.
However, it is understood that there will be no differential grading based on the regional impacts of the pandemic.
Additional contingency papers will also be run to give students a second chance to sit a paper if the main exams or assessments are missed owing to illness or self-isolation, the Department for Education said.