New rules for allocating top grades in GCSEs have been floated by the exams watchdog Ofqual.

From summer 2017, pupils will sit new GCSEs in English and maths, which will be graded using a numerical scale (1 to 9) instead of lettered grades (A to G). The new top mark will be grade 9, which will replace the current A*.

Ofqual was planning to award the new gold-standard grade nine to the top 20 per cent of pupils achieving a grade seven, which is roughly equivalent to the current A-grade.

But a consultation launched today proposes a new formula to determine the proportion of pupils receiving top grades in each subject to account for variance in the number of high-achievers sitting each exam.

Research by Education DataLab shows that a universal rule for the deciding the proportion of top grades across all subjects is difficult as the “underlying academic capability of students sitting different subjects is not the same.”

The organisation modelled three different ideas for designating top grades: the original 20 per cent plan; a tailored model proposed in Ofqual’s consultation; and a third system in which half the pupils currently receiving A*s receivex a grade nine.

It found Ofqual’s original 20 per cent approach would result in more grade nines in some subjects than the current proportion of A*s – even though the grade nine is supposed to be more difficult to achieve.

Under the formula now proposed by Ofqual, the proportion of pupils getting a grade seven in a subject will be divided by two, then seven percentage points will be added to reveal the proportion of pupils receiving the top grade.

For example, if 22 per cent of pupils achieved a grade seven in music, the number would be divided by two to give 11 per cent. Seven percentage points would then be added, to give a figure of 18 per cent. This would mean 18 per cent of top scorers among the grade seven achievers would receive a grade nine.

Ofqual intends to award grade eights in such a way that there will be an equal space between grades seven and nine.

The rule will only be applied in the first year of the new GCSEs, after which the boundaries will be based on a “mixture of statistics and examiner judgement”.

An Ofqual spokesperson said: “The proposals are designed to protect students taking the new qualifications, particularly in the first year when teachers will be less familiar with the new content and how it is assessed. The aim is to minimise unexpected or unfair outcomes for students in the transition to the new GCSEs.

“We are proposing to adopt a modified approach to the award of grades eight and nine across all subjects, including English language, English literature and mathematics. We would not normally suggest adapting a previously announced decision but we believe it is the fairest outcome for students.”

The consultation is open until June 17.