High-flying students at England’s most elite private schools are turning away from foreign-language A levels because of the “severe and unpredictable” grading of the exams, a leading teacher has warned.

Nick Mair, chair of the Independent Schools’ Modern Languages Association (ISMLA), spoke out as leading schools in the private sector described the lack of A* grades in some of this year’s language exams as “mindboggling” and even “farcical”.

Mr Mair said it was “extremely urgent” that the exam regulator, Ofqual, and boards carry out a promised investigation into the issue, before students deserted the subjects in favour of those with a greater chance of scoring the top grade.

He warned that private schools, renowned for their high participation levels in languages, were already reporting falls in uptake at A level.

“We know there are schools where pupils are receiving no A*s in languages at all, where in other subjects up to a quarter of the cohort is doing so,” Mr Mair told TES.

“Numbers taking modern languages (in private schools) still look healthy, but the perception about not being able to get top grades is already scaring students away. We would say the decline in bums on seats in independent schools is almost entirely due to this issue.”

Combined figures from state and private schools show a dramatic fall in entries for modern languages at A level, with the number of students opting for French declining by nearly 40 per cent since 2001.

Taken alone, the drop at private schools has been less dramatic, but figures from the Centre for Evaluation and Monitoring (CEM) at Durham University show that the total number of students gaining A levels in French, German or Spanish fell from an average of 16.2 per private school in 2002 to 14 in 2012.

The main concern for the ISMLA is the proportion securing the top grades needed for entry into leading universities. Figures from 2012 highlighted by the organisation show that the percentage of A* to A grades in French was just 17 per cent. This compares with 40 per cent in mathematics and 30 per cent in all subjects.

The organisation has reported particular concern among teachers about the erratic marking of oral exams and literature papers, and harsher treatment of students who are native speakers of the language they are being examined in.

The ISMLA has received a wave of complaints from the heads of some of the country’s most successful school language departments after the publication of A-level results last month.

One writes: “The German situation seems farcical. It is just not realistic to think that intelligent, well-prepared native speakers cannot score above 90 per cent. Not if this is an examination designed for non-native speakers two years on from the dizzy heights of GCSE.”

They add that in the oral exam, to score an A* means that “you cannot think before you speak”.

“Anyone who is a deliberate speaker is penalised, even if that is their natural way of speaking in their first language,” one respondent says.

Another senior languages teacher writes: “The German marking is abominable, with three native speakers (among them one Oxbridge entrant) rated as A. (It is) mindboggling.”

Mr Mair told TES: “Teachers are concerned and do not know what to expect. Independent schools are not immune to the decline in numbers of those studying languages.”

Tim Hands, chairman of the Headmasters’ and Headmistresses’ Conference of private schools, backed up Mr Mair’s concerns, saying schools were reporting that students were increasingly reluctant to take languages beyond AS level.

“There isn’t any doubt that unless there is a prompt investigation into the discrepancy between A*s awarded in languages and other subjects ... then students will turn away from languages,” he said. “With the A* being increasingly a requirement, people do not want to jeopardise their places.”

Ofqual revealed last month that it will be investigating the relatively small proportion of A* grades awarded in foreign languages to ensure that “standards are as comparable and consistent as possible”.

Exam boards have also announced plans to investigate the matter. Andrew Hall, chief executive of the AQA board, said that relative difficulty was “a real issue”. “In languages, there is a problem,” he said.