Exams may not be run for some GCSEs and A-levels qualifications this autumn term, Tes has learned.
The news means that Ofqual's pledge that candidates unhappy with any teacher-assessed grades they get this summer can sit an exam and get new grades "before Christmas" may not be fulfilled.
The uncertainty arises because the regulator is considering leaving it up to exam boards to "decide which exams to make available to students in the autumn series".
Tes has approached England's big three exam boards and none of them are committing to offering exams in all their GCSEs and A levels, if it was up to them to decide.
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If exam boards decide not to offer exams for all their GCSEs and A levels then Ofqual will have to backtrack on the pledge it made to pupils earlier this month, when it said that: "Students who do not feel their calculated grade reflects their performance will also have the opportunity to sit an exam in the autumn."
The Association of School and College Leaders (ASCL) has told Tes it would "favour" seeing exam boards only being compelled to offer autumn exams for A levels and English and maths GCSEs.
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There have been concerns that teacher bias could unfairly disadvantage some groups of students in the teacher-assessed GCSE and A level grades they get this summer.
This month, racial equality think tank The Runnymede Trust wrote to education secretary Gavin Williamson warning that teacher-assessed grades could be prone to bias against disadvantaged young people and those from black and minority ethnic (BAME) backgrounds.
This summer's "exceptional" arrangements will not involve Ofqual making any attempt to look for ethnic, gender or class bias within schools' teacher-assessed GCSE and A-levels grades.
There will also be no right of appeal against teachers' professional grading judgements.
So the only option for redress available to candidates who felt they had been unfairly graded would have been an autumn exam.
But now, having made that offer, Ofqual has warned of "a number of risks associated with compelling exam boards to make qualifications available in all subjects" for autumn exams.
It is concerned about "the costs for exam boards of providing exams for which there are very small numbers of entries".
The regulator is also considering the impact of autumn exams "on teachers who are assessors, students taught by teachers who are assessors and centres [schools] who will need to release teachers to be assessors".
"We are mindful that the autumn series will likely take place during the first term for a new cohort of students – who might have been away from school for a prolonged period – when teaching resources will be stretched," Ofqual says.
"In these circumstances, it might be beneficial to the system as a whole to allow exam boards to reduce the number of exams taking place, and the consequent impact on teachers who are also assessors, even if this does detrimentally affect some students."
But the watchdog also recognises that allowing exam boards not to offer autumn exams in all their GCSEs and A levels would "undermine the value of the autumn series in mitigating any unfairness which students and centres might consider arises from our proposals in respect of calculated grades and appeals".
"The autumn series will only provide an opportunity for students to demonstrate their abilities in an exam to the extent that exam boards decide to make those exams available," it adds.
Ofqual says it will need to conduct a further consultation on the issue, but it also notes: "Unless we decide otherwise, the default position will be that exam boards will decide which exams to make available to students in the autumn series."
None of the three major UK exam boards could confirm they would run a full complement of exams in autumn.
AQA confirmed they would be looking at the consultation and responding to Ofqual, and Pearson Edexcel referred Tes to Ofqual for information.
An OCR spokesperson said: “We want to help our students to progress with grades they can be proud of.
"We are giving careful consideration to all aspects of our response to the cancellation of this summer’s exams, including the composition of the autumn series.
"Ofqual intends to consult on proposals relating to the autumn series in due course, and we will provide our schools and colleges with more information as soon as we are able.”
Geoff Barton, ASCL general secretary, said: “We would favour a requirement that exam boards hold an autumn series in GCSE English and maths, and in A-level subjects.
"Other GCSEs are very important for progression but they are superseded by other qualifications taken later down the line, while GCSE English and maths, and A levels, have an enduring significance for the individual.
"It would, therefore, seem reasonable that in the current circumstances an opportunity is provided to sit these exams in the event that a student is unhappy with the grade awarded in the summer. However, we would emphasise that this is a back-up and that we are confident the system of centre-assessed grades will be fair, accurate and consistent.”
Mary Bousted, joint general secretary of the NEU teaching union, said she did not think Ofqual could make any "firm promises" in the current situation, but said the anxieties around the awarding of grades stemmed from "putting all our eggs in the exams basket".
"I don’t see how any firm promises from Ofqual could be made on that. We don’t know how the pandemic will play out; we don’t know whether schools and colleges will be open as centres in the autumn term," she said.
"They may only be open on a partial basis, so I don’t see how you could then say you could get 300 students in a room to sit an exam with social distancing.
"I can’t see those social distancing rules are going to be relaxed any time soon and that will be a major constraint on shovelling young people into exam halls.
"There will be students unhappy with their grade – the rank order, the calculated grade from teacher assessment – and it’s some justice they could resit them, but there’s a higher issue here, which is the safety of these pupils and whether they [schools] could manage it, and we just don’t know.
"I don’t think Ofqual is in a position to give clear answers about this – it’s something bigger."
An Ofqual spokesperson said: "There are a number of considerations relating to the autumn series where we will set out further proposals for consultation as soon as we reasonably can.
"These areas include whether we should require exam boards to make exams available in an autumn series, rather than permit them to do so."