Fewer teenagers scored at least five Cs including English and maths in their GCSEs this year, official figures show.
In total, 58.6% of pupils in England reached the threshold, down 0.8% on last year, according to statistics published by the Department for Education (DfE).
Statisticians suggested that the fall was down to a dramatic fall in the proportion of private school pupils sitting GCSE English.
Less than two thirds (63.5%) took the qualification this summer, compared with 92% in 2012. Many students at fee-paying schools are taking other English exams that are not included in the Government's measures, it was suggested.
In state schools alone, there was a rise, with 60.2% gaining five or more A*-C grades, including the basics, up 1.4% on last year, the figures show.
The latest statistics also show a rise in the number of children taking the government's English Baccalaureate.
The EBacc is a measure used in league tables that is awarded to pupils who achieve at least a C at GCSE in English, maths, science, history or geography and a foreign language.
More than a third (35.4%) of state school pupils entered for the EBacc this year - up from just under a quarter (23.1%) last year.
The DfE said that this equates to an extra 72,000 children taking the EBacc this year.
But there was a fall in the proportion of pupils who were entered for the EBacc who went on to achieve it, the figures show.
Overall, 64% of all those entered for the required subjects achieved the measure compared with 70% in 2012.
There were rises in the numbers of pupils taking GCSEs in more than one science, languages, history and geography, the data shows.
Pupils who took their GCSEs this summer were the first year group to do so after the Government announced it was introducing the Ebacc.
The figures also show differences remain between the genders, with girls outperforming boys.
In the Ebacc, the gender gap widened, with 39% of girls entering all the required subjects, compared with 30.4% of boys - a 8.6 percentage point gap. Last year the gap was 5.1 percentage points.
Around 64.1% of girls scored at least five C grades at GCSE, including English and maths, compared with 53.3% of boys.
Education Minister Elizabeth Truss said: "We have reversed the long-term decline of the key academic subjects that give children the best chance to get on in life.
"For years children were steered away from subjects like languages and history but the EBacc is fixing that.
"Pupils who study these subjects have more options, especially if they come from poorer backgrounds."
Separate figures show that fewer sixth-formers gained A* and A grades in three A-levels this year.
In total, just over one in eight (12.1%) got at least three top grades, down 0.7 percentage points on last year.
At A-level boys did better than girls, with 12.6% of male students gaining three A*-A grades, compared with 11.7% of their female classmates.
Private schools also continued to dominate, with three in 10 (29.5%) of privately-educated students scoring three A*-A grades, compared with one in 10 (10.5%) of those at state schools, and 7.7% of those at further education colleges.
Record numbers of pupils are taking A-level science courses, the DfE said, while more girls are entering for maths and further maths.
The statistics also show that 14.9% of sixth-formers scored at least two A grades and a B at A-level, including at least two "facilitating subjects".
These are subjects required most often by Russell Group universities, which are considered among the top institutions in the UK. The subjects include English literature, maths, physics, biology, chemistry, geography, history and languages.
The Russell Group has previously said that A-level choices do matter and that some subject combinations can help students keep their degree course options open. But it has added that this is not the only measure used by universities when deciding on admissions.
Earlier this week, Mike Nicholson, director of undergraduate admissions at Oxford University, suggested that Russell Group advice on facilitating subjects had been misinterpreted, and it was not the case that having three A-levels in these subjects would guarantee a place at one of these universities.