There will be optional assessments set by exam boards for all subjects, but they will not be taken in exam conditions or decide final grades.
Natalie Perera, chief executive of the Education Policy Institute (Epi) think tank, said: "Without timely and detailed guidance for schools on how this year's grades should be benchmarked against previous years, and with classroom assessments only being optional, there is a significant risk that schools will take very different approaches to grading.
"This could result in large numbers of pupils appealing their grades this year or extremely high grade inflation, which could be of little value to colleges, universities, employers and young people themselves."
Other teachers, however, have welcomed Education Secretary Gavin Williamson's expected move to give schools wide flexibility in deciding how teachers assess their pupils, based on those parts of the curriculum they have been taught.
Watch: Williamson on exams: ‘There’s going to be no algorithms’
Leora Cruddas, chief executive of the Confederation of School Trusts, said: "There is a reasonable consensus that teacher judgment will need to be both supported, scaffolded and quality assured.
"This is because although the pandemic has had a damaging impact, we still want assessment outcomes this year to reflect something objective."
Paul Whiteman, general secretary of the school leaders' union Naht, said the plans "appear to chart a path which avoids the awful chaos of last year".
He said: "This set of decisions is, however, only the starting point. It is now down to the awarding bodies to provide the detail which schools and colleges need to implement the process.
"Although earlier results for students seeking to start university could be beneficial, cramming GCSE results into the same week will place unnecessary pressure on to the system."
Mary Bousted, joint general secretary of the National Education Union, said the final decisions were "better" than the original proposals, adding it was "likely the least worst option available".
But she added: "However, there are still question marks over how it is expected that the extra work necessary to facilitate grading will be dealt with."
Last summer, thousands of A-level students had their results downgraded from school estimates by a controversial algorithm before Ofqual announced a U-turn which allowed them to use teachers' predictions.
This year exam boards will provide teachers with optional assessment questions for students to answer to help schools decide which grades to award, after this summer's exams were cancelled due to the pandemic.
But these assessments are not expected to be carried out in exam conditions and teachers will have the flexibility to choose how long students have to complete the task, and where it will be carried out.
Concerns have been raised that the Government's approach to grading A-level and GCSE students could result in "extremely high grade inflation".
The Department for Education and England's exams regulator Ofqual have confirmed teachers will be able to draw on a range of evidence when determining grades - including mock exams, coursework or other work completed as part of a pupil's course, such as essays or in-class tests.
Pupils will only be assessed on what they have been taught after months of school and college closures.
Schools and colleges will submit their grades to exam boards by June 18 to maximise teaching time, and students will receive grades in early August once quality assurance checks have been completed by the exam boards.
Normally students receive their results in mid to late August, but A-level students will receive their results on August 10 and GCSE pupils will receive theirs two days later on August 12.
It is hoped the earlier schedule will ensure pupils have enough time to log appeals so A-level students do not miss out on their preferred university places for the autumn.
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