Data released in parliamentary answers, and research into the exams chosen by private schools, shows the extent to which the independent sector is still opting for less demanding, internationally-recognised GCSEs (IGCSEs), which state schools are progressively being barred from using.

Labour MP Lucy Powell, a former shadow education secretary, who received the data after tabling a parliamentary question, said it was now clear that reforms of the GCSE system had put state school pupils at a disadvantage compared with their private school counterparts.