The proportion of GCSEs awarded at least a C grade has fallen for the first time in the exam's history, as a row broke out over English results.
Many schools are reporting a drop in the number of children scoring a C or above in the key subject, with headteachers claiming that this summer's English exams were marked too harshly. They have suggested that exam boards substantially increased grade boundaries halfway through the year amid fears that too many children were going to get a C.
Brian Lightman, general secretary of the Association of School and College Leaders, called for an investigation into the matter and accused the Government of yearly "tinkering".
He said: "The big issue, schools are telling us, is at the C/D borderline in English. What has happened this year is not that the exams or standards have been made more rigorous in a way that young people and teachers can prepare for. What appears to have happened is that, halfway through the year, it was decided that too many students were going to get a C grade in English and the grade boundaries of the exam were pushed up very substantially."
He added: "Standards in schools have not changed one iota. It is the grading that has changed. It is morally wrong to manipulate exam grades in this way - you are playing with young people's futures."
But Education Secretary Michael Gove insisted that any changes in grades were the result of "independent judgments made by exam boards entirely free from any political pressure". He said the reason some pupils have had poorer results than expected was partly down to a change in the system which meant their exams had been split into units and modules this year.
The national results revealed that 69.4% of all GCSE exams were given at least a C grade - down 0.4 percentage points on last summer. It is the first time the A*-C pass rate has fallen in the 24-year history of GCSEs. The exams were first taught in 1986, with the first exams taken in 1988.
The results also show that the proportion of entries awarded at least a C in the key subjects of English, maths and science have fallen. The national figures show that in the English GCSE, 63.9% of entries got at least a C, compared with 65.4% last summer, while 15% were awarded an A or A*, down from 16.8% in 2011.
Exam chiefs said that the falls in English were partly down to more candidates switching from taking the subject early in the winter exam season. They said that historically, there had always been significant numbers of re-sits after the winter exams, and that the winter season tended to produce lower results. As more students move to take their English exam in the summer, this has an impact on results.
Labour said the Commons Education Committee should now carry out an inquiry into what happend. Shadow education secretary Stephen Twigg said: "Michael Gove needs to take responsibility for the exam system and not just pass the buck. If pupils from the same year group have been marked more harshly if they were entered later, that is patently unfair. We need to understand why some pupils' grades have fallen so dramatically."