Exam results published over the past two weeks have shown a big drop in numbers taking French and German.
Further falls were predicted in future years following the Government's decision to make studying a language optional for 14-year-olds from 2004.
Ministers are concerned at schools' reaction, The TES has learned. Stephen Twigg, education minister, has written to local authorities urging schools not to cut language provision.
The letter, which was also signed by Lid King, the national director of languages, said that schools were reducing languages provision "in some cases quite significantly".
It said: "Schools remain under a statutory requirement to offer languages at key stage 4 so that pupils have an entitlement to take languages as an option.
"They have no entitlement if schools have effectively dropped languages from the post-14 curriculum or restricted their availability in ways which make it difficult for pupils to choose languages."
Official research, to be published next month, shows that four out of five local education authorities believe they will not meet the Government's targets for primary language learning. By 2010, all schools are expected to offer junior children the chance to learn languages to the same standard as other subjects.
Initial findings from research commissioned by the Department for Education and Skills show only 31 of England's 150 LEAs believe that more than 80 per cent of seven to 11-year-olds will get the chance to learn a language by the end of the decade.
And 41 per cent of teachers - including 8 per cent of those already giving language lessons - said they were not confident about learning a language, .
However, the research did show that the proportion of primaries offering language lessons has almost doubled over the past four years to 44 per cent.
Teresa Tinsley of CILT, the National Centre for Languages, said: "If LEAs have not started planning for this yet, then they should if the entitlement to languages is to be in place by the end of the decade."
Pupils taking French GCSE dropped 4 per cent to 331,089 this year, while German entries slumped 3 per cent to 122,023, despite a 3 per cent rise in the number of 15-year-olds.
The figures will worsen next year. A TESCILT survey in the autumn of 2002, when this summer's GCSE pupils were embarking on their courses, found 29 per cent of schools had dropped languages at key stage 4.
A year ago, the figure rose to 45 per cent, indicating that a further slump can be expected next year. The subject does not officially become optional until next month.
An adviser to Charles Clarke, the Education Secretary, said that ministers would be looking to Mike Tomlinson's review of 14-19 qualifications to address the issue in secondary schools. But the review is not proposing to reverse the decision to make languages optional from 14.
Michael Fluegger of the German Embassy in London said: "It is regrettable that in this country language learning for older pupils is optional."
A spokeswoman for the DfES said: "Almost half of primary schools are now offering language learning opportunities compared to 20 per cent in 2001."