The Labour party says GCSE reforms in England are putting state school pupils at a disadvantage by forcing them to sit harder exams than students in the private sector.
The party is also demanding an inquiry into the situation.
The Department for Education describes the reformed GCSEs, which were introduced last year, as “gold standard”, but data shows that many independent schools are opting for internationally recognised GCSEs (IGCSEs), which are being phased out of state schools.
Critics say that this means private school pupils are being given an over state school students in the competition to win university places.
The shadow education secretary, Angela Rayner MP, said: “We cannot have an education system with different rules for the privileged few. It is totally wrong that Tory reforms are putting state school pupils at a disadvantage compared to their counterparts who can afford a private education.
“We urgently need to get to the bottom of this situation. A full, root-and-branch review of Tory reforms to qualifications and their impact on pupils is needed.”
The figures show that 91% of the 26,042 IGCSE entries in core (EBacc) subjects such as English, Mathematics and the sciences were in independent schools this year. Independent schools educate just 7% of all pupils in England.
“We cannot have an education system with different rules for the privileged few. It is totally wrong that Tory reforms are putting state school pupils at a disadvantage compared to their counterparts who can afford a private education."
IGCSEs were approved for use in state schools in 2010 but were removed from league tables in 2014. According to the Department for Education, once the first exams are held in a reformed GCSE subject, the equivalent IGCSE is removed from key stage 4 performance tables, meaning state schools cannot use them.
Labour's Lucy Powell - whose office analysed the IGCSE figures, and is a former shadow education secretary - said: “State school pupils have been treated like guinea pigs while many independent schools have gamed the system, insulating their institutions and their pupils against these changes, keeping the easier international GCSEs completely, or waiting for the new GCSEs to bed in before opting to enter their pupils on to these courses.”