The proportion of teenagers scoring at least five Cs at GCSE including English and maths has fallen for the first time, official figures show, adding fuel to the English exam results debate.
In total, 58.6% of pupils in England achieved five A*-C grades, including the two key subjects, down almost half a percent on 2011, according to government data.
Statisticians said the drop was down to fewer English entries from private schools, but there are also likely to be concerns that issues with this year's GCSE English grading may have played a part.
The figures show that one in four pupils were entered for all the subjects in the Government's English Baccalaureate, with 18.1% achieving the EBacc.
This is up on last year, when 23.8% entered and 17.6% achieved it.
Teenagers are awarded the EBacc if they score a C grade or higher in English, maths, science, history or geography, and a language.
The news comes as the debate over whether this year's students were unfairly marked in their GCSE exams. Headteachers claim the results were a "gross injustice", while other students say the fiasco has had a monumental impact on their lives. Although students have been offered a re-sit, many are still campaigning to have the exams re-marked.
Government officials said there has been an increase in overseas students attending UK fee-paying schools, and many of these either do not take English, or take English as a second language, which is a different subject, and this accounts for the drop.
Russell Hobby, general secretary of the National Association of Head Teachers, said that this year's GCSE results had been overshadowed by the GCSE English "shambles".
Hobby said: "Having seen grade boundaries moved between January and June, and papers regraded in Wales but not England or Northern Ireland, it is our feeling that the drop in the number of students getting five A*-C grades including English and mathematics is related to this.
"We can see from the data that schools are continuing to make excellent progress but, nevertheless, thousands of young people have had their results and their futures hampered by the GCSE marking fiasco.
"We continue to prepare the framework for a legal challenge to see these wrongs righted, with the aim of English being regraded, and we are optimistic in wanting to see official data which will reflect this regrade in the near future."
The figures show that in state secondary schools alone, there has been a 0.1 percentage point rise in the number of pupils getting five good GCSEs.
A Department for Education spokesman said: "It is welcome that more students are studying the core academic subjects that will open more doors to them for their future.
"The EBacc is the platform for young people to go on to A-levels and high-quality vocational study, and is helping us compete with leading nations who expect all students to study a rigorous academic core."
Verity O'Keefe, employment and skills adviser at EEF, the manufacturers' organisation, said: "Today's drop in the number of young people achieving five good GCSE passes, including English and maths, shows that we are still some way off the mark the economy needs to grow the pipeline of people with good basic skills.
"Many employers use similar benchmarks when recruiting young people, with three-quarters of manufacturers prioritising attainment in maths, English and the sciences when recruiting apprentices.
"However, a lack of attainment in key subjects is still restricting firms' ability to fill vacancies and government must now set a target of 65% of students achieving five good passes which includes English and maths."