The proportion of GCSEs awarded at least a C grade has fallen for the second year running, official figures have revealed.
Just over two-thirds (68.1%) of entries scored A*-C this summer, down 1.3 percentage points from 2012 - the biggest fall in the exam's 25-year history.
The proportion of entries gaining top grades has also fallen by 0.5 percentage points - with 6.8% achieving a coveted A*.
Girls continued to out-perform boys, scoring higher results at A* and A*-C across all subjects.
The dip in national results comes amid major upheaval in the exams system and it is thought that a rise in pupils entering maths GCSE early or multiple times, changes to science GCSEs and an increase in pupils taking international GCSEs (IGCSE) in certain subjects have contributed to the falls in performance.
The latest statistics show a drop in the proportion of entries scoring at least a C in key subjects including English, maths and science.
:: In English, 63.6% of entries gained a C or higher, down from 64.1% last summer.
This comes amid a rise in the number of younger students taking GCSE English, the Joint Council for Qualifications (JCQ) said. This summer there were 61,000 more entries for the subject and of these more than two-fifths (41.9%) were from pupils aged 15.
:: In maths, 57.6% of entries scored an A*-C grade, compared to 58.4% in 2012.
JCQ said there had been "significant early and repeated entries" for maths, with more than half a million entries before the summer exams. While results for 16-year-olds - the age at which pupils usually sit GCSE exams - remained "virtually unchanged", there was a decline in performance among 15-year-olds - those who are taking the exam early.
:: There has been a 7.6 percentage point fall in the proportion of entries awarded a C or higher in GCSE science. It follows a move by Ofqual to toughen up the qualifications after a 2009 report by the regulator found that the courses were too easy. This is the first summer that results have been given for the revamped GCSEs.
More pupils were entered for the three separate sciences - biology, chemistry and physics - but there was also a drop across the board in entries scoring decent grades. In biology, 89.8% of entries got at least a C, down from 92.6% last year, in chemistry 90% of entries scored A*-C, down from 93%, and in physics 90.8% reached this standard, down from 93.2%.
The decline in results for the separate sciences is partly down to bright students switching to IGCSE courses and an increase in the number of 15-year-olds, who tend to perform less well, taking the exams early, JCQ said.
It added that a general trend of more students opting for the three sciences, some of whom will have lower abilities in the subject, may also have had an effect.
:: The results also show a "dramatic" rise in the number of entries for modern foreign languages. French entries are up 15.5%, German up 9.4% and Spanish up 25.8%.
This could be down to the introduction of the Government's English Baccalaureate, which is awarded to pupils who score at least a C at GCSE in English, maths, science, history or geography, and a foreign language.