German could disappear from schools completely and there is a "great danger" of the UK failing to produce enough good linguists, teachers from the independent sector have said.
They warned of a "crisis in modern foreign languages" as a report suggested that inconsistencies in grading have been more pronounced than ever this year.
Able pupils were being put off languages, the teachers said, because they did not believe they could score the highest grades.
A report by the Headmasters' and Headmistresses' Conference (HMC) and the Independent Schools Modern Languages Association (ISMLA) found that no school was entirely happy with its results this year, with a "flawed" grading system meaning the most able students were the most likely to be penalised.
The report said: "German is now vulnerable to withering away completely."
The concerns have prompted William Richardson, general secretary of the HMC, to write to admissions tutors to warn them that schools no longer have confidence in the grades their students are getting.
He said top schools were worried that their most able students were “not properly rewarded” in language A-levels.
“This year, for the first time, we’ve started to write to admissions tutors to say, when making your offers, schools no longer have confidence in the grades students are getting and you need to consider that when looking at the Ucas applications,” he said.
Peter Hamilton, chair of HMC's academic policy committee, said schools were telling universities that “although you made an offer which said A or A* in German, and it applies to other languages, if the person turns up with a grade below, please don’t think it’s because they’re an abject failure.
"Nine times out of 10 it’s going to be because the [exam] wasn’t properly marked or properly set.”
Mr Hamilton said teachers who knew their pupils' language ability very well, from speaking to them and carrying out internal assessments, "find that the person you thought was top usually comes near the bottom, and so on and so forth".
"It really is a higgledy-piggledy mess,” he added.
Mr Hamilton said that able pupils believed they would not be fairly assessed and so opted to do other subjects. "The bottom line is that this country is in great danger of not producing any decent linguists at all in the future," he added. "It puts us at a disadvantage globally.
"To have a generation of people going out there with only a handful that have got that sort of flexibility, that cultural and linguistic flexibility, is going to put us really, really behind the times.
"We will just drift into oblivion."
The HMC, which is meeting in St Andrews this week, said it is calling on exam regulator Ofqual for further reform of question-setting and grading.
But a spokesperson from Ofqual said the watchdog was "surprised and disappointed" by Mr Hamilton's comments.
They said: "We have worked closely with HMC, and other interested parties, since last summer to address identified issues in these subjects.
"This work resulted in significant changes to the design and approach of exam board papers ahead of this summer’s exam series."