The Labour party is demanding an inquiry into GCSE reforms in England that it says are putting state school pupils at a disadvantage by forcing them to sit harder exams than students in the private sector.

The Department for Education describes the reformed GCSEs, which started to be introduced last year, as “gold standard”. But official figures show that many independent schools are opting for internationally recognised GCSEs (IGCSEs), which are being phased out of state schools at the behest of the government because it considers them less robust.

The consequence, according to critics, is that private school pupils are being afforded an advantage over state school students in the race for 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, released in response to parliamentary questions tabled by the Labour MP Lucy Powell, 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 7% of all pupils in England.

IGCSEs were approved for use in state schools in 2010 but they were removed from league tables in 2014. According to the DfE, 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.

The reformed GCSEs have faced criticism for causing increased stress and anxiety to pupils by scrapping coursework and modules and making exam performance more crucial. But Labour says that private school pupils are not being held to the same standards.

Powell 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.”

Analysis by her office of the highest-performing independent schools showed that none puts their pupils in for the full suite of reformed GCSEs in EBacc subjects, which also include geography or history and a language.

Pupils at Eton college, where fees are £40,668 per year, were only entered into IGCSEs in EBacc subjects. At the £37,296-per year boarding Sevenoaks school and £17,040-per year Abbey school in Reading, students were only entered for IGCSEs.

Julie Robinson, general secretary of the Independent Schools Council, said schools in the private sector chose qualifications “based on the intrinsic educational quality of the course content. They have a responsibility to ensure pupils are fully prepared for their next steps in life, achieving qualifications that are well-respected and valued by universities and employers”.

“More than half of exam entries in 2018 were for GCSEs, while IGCSEs – taken by millions of students around the world – accounted for the remainder. Historically, independent schools opted for IGCSEs due to their rigour and reliability compared with GCSEs, before the introduction of the recent reforms, which are still being rolled out.”

IGCSEs include coursework, whereas the Association of School and College Leaders, which has expressed concerns about the impact of the reforms on pupils’ mental health and wellbeing, says year 11 students typically sit more than 30 hours of examinations in the new GCSE system. The reforms were introduced by the then education secretary Michael Gove in an attempt to make courses more demanding and in response to perceived grade inflation.

The DfE has been approached for a comment.

• This article was amended on 31 December 2018 to clarify that the GCSE reforms only apply in England.