Two out of five teenagers who sat essay-based A-levels may be awarded the “wrong” grade when results come out on Thursday because of inconsistent marking, according to research by the exam regulator Ofqual.

The study shows the probability of a candidate not getting the correct grade in subjects such as English and history is between 42% and 48% because examiners mark subjectively. For sociology it is 37% and for geography 35%.

The findings threaten to undermine the reputation of A-levels as the education system’s gold standard after its biggest shake-up in a generation. Results are expected to be good again on Thursday with a pass rate of about 98% and more than a quarter of entrants gaining top A* or A grades. But for the