Harsh grading is resulting in a decline in the number of students sitting modern foreign languages, with native speakers performing less well than those whose mother tongue is English, it has been claimed.
Independent school headteachers said students sitting Spanish, French and German from GCSE through to A-level had been marked more heavily for the last decade, compared with other subjects.
Members of the Headmasters' and Headmistresses' Conference (HMC) said poor exam results were "sapping (students') confidence", while entries in A-level Spanish, German and French are all down on the previous year, by 2.7%, 4.2% and 6.4%, respectively.
James Priory, headteacher at Portsmouth Grammar School, said: "We have seen unpredictable language results this year. A number of students predicted B grades, for instance, have received grades below expectation, with the result that they are no longer set on studying languages at university.
"Just as baffling, high-achieving bilingual students are performing less well in the oral than those whose first language is English.
"We have reached the point where the marking and grading of our International Baccalaureate linguists is becoming more reliable year on year than those of those students sitting language A-levels."
Latest figures from the HMC show its students were now seven times more likely than state school pupils to study modern foreign languages at university - up from five times last year.
William Richardson, HMC general secretary, said: "It's not just a sickly subject, but really getting towards intensive care now."
It comes as the HMC announced a new Government-backed national teacher training centre to encourage greater take-up of the subject.
HMC chairman Mike Buchanan, speaking at the HMC conference in Stratford-upon-Avon, said the initiative would work by identifying schools around the country which would be willing to train teachers.
Poor grades have been identified as one of the main causes for deterring students from sitting "high-risk" subjects, resulting in fewer languages teachers.
Mr Buchanan said: "For over 20 years, educationalists and the Government have been concerned about the decline in numbers of students choosing to study languages. This is not only culturally impoverishing, but likely to put UK pupils at a major disadvantage in a global marketplace in which 75% of people do not speak English.
"The reasons for this are complex but include the difficulty of achieving a top grade compared with other subjects, leading to less take-up, smaller departments and fewer teachers.
"The details are being worked out, but there is huge energy behind the project from all the partners."
Schools minister Nick Gibb said: "The UK now more than ever must be an outward-facing nation.
"We must endow the next generation with the ability to read, write and speak modern foreign languages."