Exams regulator Ofqual has explained what constitutes a "valid" mock exam for the purpose of students appealing against A-level results in England.
Thousands of grades were marked down after a moderation process used in place of this summer's exams.
The regulator says the system had led to many students feeling disappointed and "results which need to be queried".
Ofqual now says where a written mock exam was not taken it will consider other teacher assessments instead.
Neither A-level nor GCSE students were able to sit public exams this year because of the coronavirus pandemic.
Thursday's A-level results saw almost 40% of all grades marked down from teachers' predictions.
Ofqual confirmed appeals using mock results could begin from Monday and would apply for GCSE, AS and A-level students as well as those taking Extended Project Qualifications and Advanced Extension Award in maths.
The government had already announced any school could query a final grade if it was a lower that a student's mock exam.
But with schools shutting down in March, there was a lack of clarity over what constituted a mock exam with some students complaining they did not get a chance to sit one.
'Face-saving exercise'
Ofqual says it will allow a "non-exam assessment mark" as the grounds for an appeal.
Geoff Barton, general secretary of the Association of School and College Leaders, criticised the process, branding it "surreal and bureaucratic".
He said: "This is clearly a face-saving exercise by a government which has said that it won't do a U-turn on its pledge that moderated grades will stand, come what may.
"Instead, it is attempting to remedy the grading fiasco through an appeals process so surreal and bureaucratic that it would be better off at this point doing that U-turn and allowing original teacher-assessed grades, where they are higher, to replace moderated grades."
He added: "We don't blame Ofqual for the bizarre nature of the appeals criteria. The regulator has been given a hospital pass by a Government that is in disarray. It is time for ministers to stop the chaos and fall back on teacher-assessed grades rather than prolong this nightmare."