The GCSE pass rate has fallen for the first time, with English, maths and science results all down on 12 months ago.
National figures show that 69.4% of all GCSE exams were given at least a grade C, down 0.4% on last summer and the first drop since the exam was introduced in the 80s.
Across the 600,000 teenagers in England, Wales and Northern Ireland, there was also a fall in the proportion of top grades handed out.
Some 7.3% of entries were given an A*, down 0.5 percentage points on 2011, while 22.4% were handed at least an A grade, down 0.8 percentage points.
The results have sparked a row over the introduction of tougher exam standards as teachers complain grade boundaries have been hiked too dramatically.
Education Secretary Michael Gove says the new approach was "fair".
He told Sky News: "The fall in particular pass rates today is a consequence of individual exam boards making tough decisions about where they are going to set grades and also the regulator making sure standards are maintained over time.
"I think it's quite right that the regulator, whose job it is to police these standards independently, should be as tough and fair as possible."
The number of pupils awarded at least a C grade in the core subjects of English, maths and science are all down.
Some 63.9% achieved at least a C in English, compared to 65.4% in 2011, and 15% were awarded an A or A*, down from 16.8%.
In English Literature, 76.3% of exams were given A*-C - down from 78.4% last year and 23.2% were given As, down from 25%.
A*-C grades in maths fell 0.4% to 58.4% and A* and A grades in the subject were down 1.1% to 15.4%.
In science, 60.7% exams were marked A*-C, down from 62.9% and 9.8% were given As or A*s, down from 11.6% in 2011.
The gap between girls and boys stalled at the very top grades, with 18.9% of boys' entries achieving an A* and A, compared to 25.6% of girls' entries - a gap of 6.7% and the same as 2011.
At grades A* to C, girls are pulling away, with 65.4% of boys' entries attaining that level, compared to 73.3% of girls' entries. Last year, it was 66% to 73.5%.
There was good news as the long decline in the take-up of modern foreign languages appears to be slowing, with smaller falls in take-up of French and German and a 10% rise in numbers taking Spanish.
But concerns have been raised that the English exams were marked too harshly, with schools reporting an unprecedented number of fails among their pupils.
The Association of School and College Leaders (ASCL) said there seemed to be a particular problem with C/D borderline grades in English.
ASCL general secretary Brian Lightman said: "We expected results to plateau because of the Government taking steps to peg them to previous levels, but we certainly didn't expect anything like this and schools have been taken by surprise."
Dozens of teachers took to the Times Educational Supplement's website to express their anger.
One who pointed out that C grade boundaries had shifted by 10 marks complained: "They've not moved the goalposts, they've put them on a different chuffing planet."
Another wrote: "Our results have been decimated. We're 10% lower than last year. Members of my department are in a state of shock as they say they've never worked harder and this is the result."
Andrew Hall, chief executive of exam board AQA, declared: "This year has got more change in it than I think I've seen in my time at any awarding body."
Adrian Prandle, from the Association of Teachers and Lecturers, said: "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."
Christine Blower, general secretary of the National Union of Teachers, added: "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. These are arbitrary changes which in no way reflect the work of students and teachers and are clearly unfair."
Shadow education secretary Stephen Twigg said: "We need to understand why results have fallen in these subjects. Is it because of pressure from Ofqual to shift grade boundaries?
"As well as ensuring standards remain rigorous, we must ensure all pupils are treated consistently and fairly."
However, John Cridland, director-general of the CBI, said: "Enhancing the rigour of our examination system will help to improve performance compared with our international competitors.
"Improving attainment in our schools is critical to the future success of our economy and society. Raising ambition and aspiration for all should be the focus of our school system."
The Joint Council for Qualifications (JCQ) insisted the drop in A*-C English results was partly down to more candidates sitting the exam earlier, during the winter exam season.
It also said science entries had risen by more than a third and blamed the fall in results partly on the increase but also on "more demanding" standards.
The tougher marking could have serious consequences for some schools, which have all been forced to ensure 40% of pupils achieve at least five good grades, including in English and maths.
Ministers have warned that schools falling short could be closed or pulled out of local authority control and turned into independent academies run by a third-party sponsor.
Failing academies could undergo a change of leadership.