The English Baccalaureate (EBacc) could disadvantage the poorest students, it was warned today as pupils collected their GCSE results.
Independent think tank Civitas said the pressure of league tables can lead to schools discouraging students from taking non-compulsory subjects if they are not expected to get a C or above in them.
To gain the EBacc, a flagship Government scheme, pupils must score at least a C grade at GCSE in English, maths, science, a foreign language, and history or geography.
While the EBacc is not compulsory, it has been widely predicted that many secondary schools will begin to steer pupils towards these subjects.
Civitas's director of education Anastasia de Waal said: "A student judged to be unlikely to get a C risks both failing to add to the league tables and being a potential distraction for teachers from the EBacc goal."
The think-tank said that because of the correlation between lower exam performance and free school meal eligibility, the students liable to be excluded from EBacc subjects are "disproportionately likely to be poorer".
Ms de Waal added: "The House of Commons Education Committee found little evidence to suggest that the EBacc would help the most disadvantaged.
"The A*-C focus is a key impediment. Ensuring equality of access to academic subjects is a positive goal, but the strategy is redundant if the most deprived lose out."
Meanwhile, exam chiefs stressed that schools must make sure pupils have a rounded curriculum in the wake of the EBacc's introduction.
Ziggy Liaquat, managing director of exam board Edexcel, said: "I think it will certainly start to drive behaviour and we're starting to see that happen already.
"The important thing to remember about the EBacc is that it does just represent part of the curriculum, because there is space and there should be space for other qualifications as part of the curriculum time.
"What's incredibly important is that we have young people with a rounded education."
The EBacc was included in league tables for the first time earlier this year, with just one in six teenagers in England (15.6%) achieving it.