The proportion of GCSEs awarded at least a C grade has fallen for the first time in the exam's history amid a row over "shifting the goal posts", official figures revealed on Thursday.
The results also show drops in the percentage of English, maths and science GCSE entries achieving passes at A*-C.
Thursday's national figures reveal that 69.4% of all GCSE exams were given at least a C grade - down 0.4 percentage points on last summer.
The NUT condemned “shifting the goal posts", saying changes to the grade boundaries were harming young people.
"In particular the C/D boundary has had a huge impact on individual students and the future of schools," Christine Blower, General Secretary of the National Union of Teachers, said on Thursday.
"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."
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.
There was also a fall in the proportion of GCSEs awarded the top grades.
Glenys Stacey, chief executive of exams regulator Ofqual said on Thursday that papers were not being harshly marked this year, saying it took time for changes to filter through the exam system.
"We are continuing to apply a measured approach to the control of standards," she told the BBC.
"These are changes that we are introducing to strengthen GCSEs. Our research shows that GCSEs and A-levels are generally well-regarded, but that doesn’t mean they can’t get better."
But dozens of teachers voiced their concerns about the issue on the Times Educational Supplement's website on Wednesday night.
One said that the score out of 80 needed to get a C grade was now a whole 10 marks higher than earlier in the year. He added: "They've not moved the goal posts, they've put them on a different chuffing planet."
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,
It comes amid reports that schools where 40 per cent of pupils do not get 5 A-C grades will face closure or being turned into academies after slipping under targets.
The statistics show a decrease in the proportion of GCSEs awarded at least a C grade in the core subjects of English, maths and science.
The Joint Council for Qualifications (JCQ), which publishes Thursday's national results, insisted that the drop in A*-C English results is partly down to more candidates sitting the exam earlier, during the winter exam season.
The number of entries for English GCSE, including English literature, has increased by 3.1%, JCQ said.
It added that there was a "dramatic" increase in entries for science GCSE - up 36.5% - and said that the fall in results at A*-C in this subject is partly due to a "more demanding standard" introduced this year, and a "significant" increase in entries by 15-year-olds.
Around 600,000 teenagers in England, Wales and Northern Ireland are receiving their GCSE results on Thursday.
Teachers have raised concerns that English exams have been marked too harshly, with schools reporting an unprecedented number of fails among their pupils.
School staff have complained that exam boards substantially increased grade boundaries, leaving pupils with lower results than expected.
English teachers who were shown early breakdowns of the marks complained exam boards had substantially increased grade boundaries, leaving pupils with lower results than expected.
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 percentage gap of 6.7%, the same as there was in 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, 66% of boys' entries achieved A* to C, compared to 73.5% of girls' entries.
The long decline in the take-up of modern foreign languages appeared to be slowing this year, with even a rise of 10% in the number of those sitting Spanish GCSE.
The number of entries for French fell by 0.5%, compared to a 13.2% fall last year, and the entries for German fell by 5.5% compared to a 13.2% fall in 2011.
There was a rise of 13.7% in the uptake of other modern languages, including Arabic, Chinese, Persian, Polish, Portuguese and Italian, which all saw significant increases.
Michael Turner, director of the Joint Council for Qualifications, said: "These are a good set of results and students and teachers should be pleased with what they have achieved.
"It will be interesting to see if this year's rise in students taking Spanish and the rate of decline slowing in French and German is the beginning of a trend that will see more young people studying languages."
Andrew Hall, chief executive of exam board AQA, said: "This year has got more change in it than I think I've seen in my time at any awarding body."
AQA stressed that the standards students have to achieve remained the same.
Ziggy Liaquat, managing director of Edexcel, said: "The quality of work required to achieve an A grade this year is the same as the quality of work required to achieve an A grade last year."
The exam boards said changes were most apparent in the science results, with Mark Dawe, chief executive of the OCR, saying that the Government wanted a more difficult paper set for students.
Mr Hall said: "There is very, very clearly a new standard in science. We said it was coming, the Government said it wanted it and we have delivered it, and it shows in the outcomes."
This year's exam results shows the impact of the new English baccalaureate, with a reverse in the decline of numbers taking history and geography, as well as improvement in the take-up of modern foreign languages.
Mr Hall said: "I think it's quite encouraging. There is still a real crisis in modern foreign languages but maybe here we see the beginning of the EBacc starting to have an impact."
Exam board chiefs said this was a year of "major change" in English, maths and science.
As well as changes to GCSE science, new English courses have also been brought in.
Mark Dawe, chief executive of the OCR exam board, said: "What's very clear is changes in the science specifications have had an impact on grades, and it was known from the beginning, the Government wanted a harder science paper, when Ofqual was accrediting it, it was a harder science paper, and when the results are coming through, this is harder."
:: In English, 63.9% of entries got at least a C, compared to 65.4% last summer, while 15% were awarded an A or A*, down from 16.8% in 2011;
:: In English literature, 76.3% of exams were awarded A*-C, compared to 78.4% last year, and 23.2% got at least an A, against 25% in 2011;
:: In maths, 58.4% of entries got at least a C grade, down from 58.8% in 2011, and 15.4% got A*-A, compared to 16.5% last summer;
:: In science, 60.7% got A*-C grades, down from 62.9% and 9.8% got A*-A, down from 11.6% in 2011.