This year's GCSE results have shown the biggest ever year-on-year decline - down to the lowest level since 2008.
The overall proportion of entries achieving A* to C has declined from 69% to 66.9%. Top A* grades have slipped from 6.6% to 6.5%.
This has been blamed in part on more pupils in England being required to resit English and maths.
Schools Minister Nick Gibb said 17-year-olds who had successfully retaken these GCSEs now had "better prospects".
Impact of retakes
The results of more than five million GCSE entries in England, Wales and Northern Ireland have been published.
This year's combined figures show a sharp decline in the proportion of grades A* to C and smaller falls in top A* and A grades.
Prof Alan Smithers, director of the University of Buckingham's Centre for Education and Employment Research, said the 2.1 percentage points decline was the biggest drop in A*-C grades since GCSEs were introduced in the late 1980s.
This has been driven by a fall in grades in England - as Northern Ireland's results improved and results in Wales remained stable.
- In England, the proportion of A* to C grades fell from 68.8% last year to 66.6%. Top A* grades were down from 6.6% to 6.4%.
- In Northern Ireland the proportion of these good passes increased slightly to 79.1% and top A* grades rose to 9.3%.
- In Wales, the level of A* to C passes remained at 66.6%, with A* grades rising slightly to 6.1%.
The fall in England has been attributed to a policy in England requiring resits in maths and English for 17-year-olds who did not make a C grade last year.
Even without these older pupils resitting exams, there has been a fall in the results of 16 year olds, with the proportion getting A* to C declining by 1.3% points.
This year's results were down for maths, English, history and geography.
And there were further falls in the numbers of pupils taking modern languages, with the British Council warning that entries for French had more than halved in the past two decades.
The National Union of Teachers said the decline in modern languages was exacerbated by the "difficulty many schools have in recruiting qualified language teachers".
The gender gap has widened further - with 71.3% of entries by girls getting a C grade or above compared with 62.4% of boys.
There has been a continuing reduction in the number of pupils taking GCSEs a year early, down by about a quarter compared with last year.
League tables now only count the first time a pupil sits an exam, discouraging entries by younger pupils.
This will be the last year before the start of a major change in how GCSEs are graded in England and how school performance is measured.
A revised set of GCSE exams are going to be graded by numbers - from 9 down to 1 - rather than the current letter-based system in a process that will be phased in from next year.
There will also be a new way of assessing schools in England, to be introduced later this year, called Progress 8, which will measure how much progress pupils make in secondary schools, rather than their raw results.
Russell Hobby, leader of the National Association of Head Teachers, said: "GCSE results day is a nervous time for all, as much depends on these grades.
"2016 is worse than ever in this regard as the results are so unpredictable following wave after wave of over hasty changes. Such overwhelming change introduces mistakes and makes it hard to sustain a calm focus on teaching," said Mr Hobby.
Labour's shadow education secretary Angela Rayner blamed "this sharp and unprecedented fall" on the government's "constant chopping and changing to assessment and the curriculum".
England's School Standards Minister Nick Gibb said he was "pleased to see that there are more GCSEs being taken in the core academic subjects, those that give students a wider range of opportunities.
"And for those 17-year-olds who have struggled to achieve good grades in maths, we are seeing 4,000 more successful re-takes of those exams; delivering better prospects for every one of those young people."
Kirsty Williams, Education Secretary in Wales, said: "This year's GCSEs show another strong performance with two thirds of our learners achieving at least A*-C and an increase in the top grades."