Professor Alan Smithers' report says almost 50,000 students could miss out on getting the A* and A grades they could have expected last year if this summer's grading returns to pre-pandemic standards.

He predicts around 10% of grades will be an A* and around 27.5% will be an A this year, compared with 2022 when 14.6% of grades were an A* and 36.4% an A.

In 2019, 7.8% of grades were an A* and 25.5% were an A.

The government has said the number of A* and A grades awarded in England should fall back to pre-pandemic levels as exams return to normal.

Prof Smithers, director of the Centre for Education and Employment Research at the University of Buckingham, expects the number of top A-level grades to fall significantly but not by quite as much as the government requested, as was the case last year.

The expert says teachers developed a "taste for awarding top grades" in some subjects during the COVID pandemic which markers will be "reluctant to relinquish".

He said: "During the teacher assessment years, many students and their parents will have developed unreasonable expectations.

"Whatever the extent to which top grades are brought down this year, the drop will lead to a lot of disappointment and probably a huge increase in the number of appeals."

Prof Smithers also said disruption from teacher strikes may have led exam boards to be more lenient.

He said the percentage of top grades in performing arts and practical subjects increased sharply during the pandemic when grades were based on teacher assessments.

That figure rose less steeply for science and maths because pupils studying those subjects were already getting more top grades before the pandemic.

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However, last year more top grades were awarded for music and performing arts than physics and chemistry despite the return of exams, suggesting a "profound change" which may make it more difficult to cut the number of top grades awarded back to 2019 levels.

This trend led to girls receiving far more A* grades than boys, who had previously had the edge because far more of them studied subjects where pupils can "manifestly amass right answers".

If more A* grades continue to be awarded for arts and humanities than before the pandemic, this could mean boys do not regain their lead.

Pupils in Northern Ireland expected to get best results

Prof Smithers said he expects pupils in Northern Ireland to get the best results, as they have done for many years, followed by those in Wales and then England.

A Department for Education spokesperson said: "This year, GCSE and A-level grading is largely returning to normal, in line with plans set out by Ofqual [which regulates exams] almost two years ago, to make sure qualifications maintain their value and students get the opportunities they deserve.

"This means national results are expected to be similar to those in pre-pandemic years, and a student should be just as likely to achieve a particular grade this year as they would have been before the pandemic.

"The number of top grades also has no bearing on the number of university places available."

A spokeswoman for Ofqual said: "This year we expect exam grades to go back to similar levels to 2019, which was the last year before the pandemic."