The finding follows the Government's decision last month to make Lord Dearing carry out an urgent review into language teaching to halt the decline.
Lord Dearing said he feared the proportion of students taking a language GCSE would slip below 50 per cent next summer.
His concerns seem to be supported by the findings from Cilt, which surveyed more than a thousand secondary schools in England. They found that the proportion of schools where more than half of Year 11 students are studying for a language GCSE had dropped from 53 per cent in 2005 to 46 per cent this year.
The number of state schools insisting that teenagers take at least one language GCSE this summer also dropped - from 29 per cent last year to 18 per cent.
Private schools fared better. But the proportion of them which make a modern language compulsory in Year 11 has collapsed from 78 per cent to only 56 per cent.
The Government abolished compulsory language lessons for 14 to 16-year-olds in 2004. Jacqui Smith, then schools minister, wrote to schools in January last year asking them to set a "benchmark" of between 50 to 90 per cent of students studying for a foreign language.
Yet the Cilt survey revealed that in 29 per cent of state schools fewer than a quarter of students are continuing with a language after the age of 14. The decline in language is most severe among schools with a high proportion of students eligible for free school meals.
Lord Dearing will not be advocating a return to compulsory languages at GCSE, but has been asked to find ways to encourage secondary schools to offer alternative courses.
The survey showed that only 22 per cent of state schools, and 9 per cent of private schools offered alternative courses - the same proportion as last year.
Teenagers in the North East are the least likely to be studying for a language GCSE. Isabella Moore, Cilt director of the National Centre for Languages, said young people needed a strong basis in languages in a competitive jobs market.
"There are advantages and there are skills which should not be the preserve of an elite," she said. Lord Dearing is due to publish initial findings next month and should complete his report by February.