GCSEs may need a grade 10, a private school chief has suggested as he revealed that at some of the country’s top fee-paying institutions the “most common” mark this year was 9.

Barnaby Lenon, chair of the Independent Schools Council (ISC), disclosed that there are now a growing number of private schools where the majority of GCSEs are given 9s, the highest possible grade.

Mr Lenon, who is a former headmaster at the £41,775-a-year Harrow School, said: “Nobody envisaged this three years ago. It does leave room for a grade 10.”

This year, almost a quarter (23.1 per cent) of private school GCSEs were awarded a grade 9.

Across the whole of England, 837 students were awarded a clean sweep of seven grade 9s, Ofqual said, compared with 732 last year. Of those awarded seven grade 9s this year, two thirds (66.4 per cent) were girls.

London’s Westminster School came top in the private school GCSE league tables, with 98.61 per cent of students achieving grades 7, 8 or 9, equivalent to As and A*s.

The school, which charges up to £41,600 a year, counts the actress Helena Bonham-Carter and former deputy prime minister Sir Nick Clegg among its alumni.

Mr Lenon said that for now, the grade 9 serves its purpose in differentiating ability at the top end of the grading system.

Mr Lenon is a former headmaster at the £41,775-a-year Harrow School

“The number of grade 9s is rationed to avoid the possibility of grade inflation anyway,” he told the Times Education Supplement magazine.

But he added that a grade 10 could be introduced in the future “if over time the general standard of pupils at the top end continues to rise”.

“Given we were told grade 9s would be very rare, it’s astonishing there are schools where it’s the commonest grade,” he said.

This summer was the first group to take the reformed courses in virtually all subjects. The new GCSEs were created by former education secretary Michael Gove as part of an attempt to inject rigour into the qualifications and bring the UK in line with top performing countries in the Far East.

The reformed exams, which are marked in numerical grades of nine to one rather than A* to G, are designed to separate the very highest achievers with the A* now split between grades 8 and 9.

More than half of students were awarded a grade 9 in certain subjects, GCSE data shows. Of those taking Chinese GCSE, 59 per cent were awarded the highest grade, according to figures compiled by the Joint Council for Qualifications (JCQ).

Meanwhile, 61 per cent of all Russian entries were given a grade 9, along with 49 per cent of those taking Ancient Greek.

A spokesperson from the exams regulator Ofqual said: "Our role is to secure that standards are maintained in GCSEs in England and we have no intention to introduce any additional grades above grade 9.

"The 9 to 1 grading system has allowed better differentiation of the highest performing students than the A* to G grades it replaced. We believe everyone would now welcome a period of stability."