French and German lessons are in "chronic decline", with too many students dropping languages altogether at age 16, a study warns.
Languages were seen as 'hard', the report says.
GCSEs in the subjects fail to provide good grounding for A-Levels, England's exams watchdog, the Qualifications and Curriculum Authority, said.
Languages are no longer compulsory for 14 to 16-year-olds.
The government said it was encouraging more interest by focusing its efforts on younger children.
During last summer's GCSEs, the proportion of candidates achieving grade C or above went up 2.5% for French, 4.2% for German and 2.8% for Spanish.
However, the number of entries in French fell by nearly 4% and for German nearly 3%. Those for Spanish rose by 4%.
The QCA said it "would not be unreasonable to assume that schools entered fewer candidates likely to gain lower grades".
Even among the pupils who did GCSE languages, teachers who took part in seminars thought there was a "considerable gap" when moving on to A-levels.
The report says: "This is daunting for students because their GCSE course has not provide them with a sufficiently broad and secure knowledge of grammar, or with the vocabulary to cope with very different topic areas."
Many teachers feared studying languages at A-Level was becoming the exclusive privilege of bright, middle-class students at independent schools.
A-level entries in languages in 2003 were fewer than those in 2001.
Take-up for French was down by 18%, German by 21% and Spanish by 5%.
Since last September it has not been compulsory to study a foreign language after the age of 14.
The report said this would "certainly" lead to a short-term decline, with evidence suggesting about two-thirds of schools had made languages optional. Foreign languages were seen as "difficult" subject at A-Level".
But the report said their status at primary school was improving as more lessons were being provided.
Educational strategy is now aimed more at interesting children in languages at a younger age, rather than trying to re-enthuse bored teenagers.
The government wants every primary school pupil to have the opportunity to learn a language by 2010.
A Department for Education and Skills spokesman said: "We are committed to increasing the take up of languages at all levels of education."