Teenagers have been congratulated for their exam results but the Government has been warned not to play "political football" with students.
Fears were expressed that the fall in the proportion of those awarded the top grades could have widespread implications for 16-year-olds and their schools.
Adrian Prandle, education policy adviser at the Association of Teachers and Lecturers, said: "There is unacceptable confusion today about whether so-called grade inflation has been banned and grade boundaries made tougher. Children's chances in life are at stake here and it is hugely unfair to make today's 16-year-olds the victims of political football.
"It is wrong to make improvement impossible. If the reason for the drop in top grades is because politicians have determined they want exams to be tougher, they must explain this to the youngsters who will suffer when they compete in the future for jobs, apprenticeships, college and university places with those a year or two older."
Chris Keates, general secretary of teachers' union NASUWT, said: "The results show a slight percentage drop in the A*-C pass rate. The Government may seek to claim that these changes are as a result of it 'toughening up' on examinations because they were 'too easy' in the past.
"This is untrue, as the evidence from the exam boards and exam regulator shows. However, the Government should consider that it is unlikely to be long before there is clear evidence that its policies are affecting educational outcomes."
Shadow education secretary Stephen Twigg said while it was important to celebrate students' achievements, questions had to be raised about the decline in English and maths.
He said: "We need to understand why results have fallen in these subjects. Is it because of pressure from Ofqual to shift grade boundaries? Concerns have been raised regarding the English GCSE in some quarters. As well as ensuring standards remain rigorous, we must ensure all pupils are treated consistently and fairly."
Christine Blower, general secretary of the National Union of Teachers, said: "Pupils, parents, carers and teachers should be very proud of today's results... Shifting the goal posts for grades, in particular the C/D boundary, has had a huge impact on individual students and the future of schools. It is not only very unsettling but also extremely irresponsible."
Professor Steve Furber, chairman of the Royal Society computing in schools report, said the 12.9% increase in the number of those taking ICT was "a good step in the right direction for a subject that has seen consistent declines for the last five years".