The proportion of GCSEs scoring a C or above has fallen for the first time in the exam's 24-year history.
Official figures reveal a 0.4% drop in the A*-C pass rate.
The results also show that the percentage of entries awarded at least C in the key subjects of English, maths and science has also fallen.
Today's national figures reveal that 69.4% of all GCSE exams were given at least a C grade - down from 69.8% last summer.
It is the first time that 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.
Exam board chiefs said this had been a year of "major change" and that drops in results were partly down to tougher science GCSEs and more candidates sitting English in the summer rather than earlier, in the winter exam season.
Around 650,000 in England, Wales and Northern Ireland are receiving their results today.
The national figures show that:
:: There was also a fall in the proportion of GCSEs awarded the top grades. 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;
:: 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.
:: The gap between girls and boys widened at grades A* to C, 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 slump in modern foreign language take up appears to be slowing - 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.
:: In England alone, 7.3% of GCSE entries were awarded an A* and 69.5% achieved a C or above. Both figures are slightly down from last year's results.
The Joint Council for Qualifications (JCQ), which publishes today's national results, said there had been 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.
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 Dawe 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."
The results were published amid a brewing row over this year's GCSE English.
Teachers have raised concerns that English exams have been marked too harshly, with schools reporting an unprecedented number of fails among their pupils.
Education Secretary Michael Gove 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.
He insisted that the decision to change the grade boundaries was down to individual exam boards and was "fairly comparable" with previous years.
He told BBC News: "You cannot have a situation where exam passes continue rising forever and ever without there being in some subjects at some points grades either falling or steadying or changing - it's just in the nature of things that inevitably there will be alterations."
Asked about concerns from English teachers that exams in their subject have been marked too harshly with many pupils who were expected to get Cs getting Ds, he said: "Yes, the number of As and A*s has fallen, but the number of Bs has increased, the number of Cs has fallen and the number of Ds has increased.
"That is a result of the independent judgments made by exam boards entirely free from any political pressure and I think that the various chief executives of the exam boards, and indeed the chief executive of the regulator Ofqual, have made it clear today that these decisions have been made because the exam boards and the regulator have sought to ensure this year as every year, that exam results are comparable over time so that we can all have confidence in the examination system."
Asked if he would like to see the old system of O-levels brought back, he said: "I would like to see GCSEs reformed, because one of the problems that we have, I think, is that the change which was made under the last government to introducing modules to having a culture of re-sits and re-takes, and indeed having too much coursework - what's called controlled assessment - and an insufficient number of marks allocated for the exams at the end of the course. I think all of these things do need to change."