Pupils from well-off backgrounds are more likely to study French and German at GCSE than those from poorer families, the analysis of exam entry patterns among 112,412 16-year-olds in 664 schools found.
In German, affluence, using free meals as an indicator, was the biggest factor affecting whether pupils took the subject, with those from advantaged backgrounds being more likely to do so.
The research, which also reported similar findings for history, was carried out on subject choices made in 1998.
There are fears that the trend in languages could be exacerbated by the Government's decision to make the subjects optional at the age of 14 from September.
The research, by academics at the University of Staffordshire and the University of Durham and funded by the Economic and Social Research Council, also found big variations in the proportions of pupils taking different subjects in different schools. In French, the figures varied from 26 to 73 per cent; in German, between 2 and 42 per cent; in history, 16 to 46; and geography, 23 to 57.
Pupils at different secondaries were making very different choices of subjects. It also found that 10 per cent of pupils, on average, did not take English GCSE, even though the subject is compulsory until the age of 16.