A dramatic fall in national GCSE results is the largest since the exam replaced O-levels, with the proportion of pupils who gained a C grade or above dropping by an unprecedented 2.1 percentage points – including a sharp decline in the numbers gaining a C or above in English.

The falls are due in large part to new government policies that force 17-year-olds who got a D or lower in English or maths last year to resit those exams, meaning more students overall were sitting the tests.

But even among pupils sitting their exams in year 11, the conventional GCSE year, the proportion gaining A*-C grades was down by 1.3 percentage points.

The national figures showed a big increase in those aged 17 or over taking the exams, driven by the government policy in England of requiring retakes. Only around one in four of those retaking the two core subjects gained a C or above, which experts said called into question the wisdom of the policy.

Mark Dawe, former head of the OCR exam board and chief executive of the Association of Employment and Learning Providers, bluntly attacked repeated retakes as counter-productive.

“Surely this is evidence enough that hitting students over the head with the same form of learning and assessment is not the way forward,” Dawe said.

“Maths and English are the most vital skills for economic and social mobility but these results show that repeating the same exercise doesn’t work.”

Kevan Collins, chief executive of the Education Endowment Foundation, agreed with Dawe that better strategies were needed to improve the skills of 16-18-year-olds who failed GCSEs.

“Simply keeping those teenagers in compulsory education for another year is not enough to ensure they leave with the skills they’ll need,” Collins said.

But Nick Gibb, the schools minister, defended the policy, saying: “For those 17-year-olds who have struggled to achieve good grades in maths, we are seeing 4,000 more successful retakes of those exams, delivering better prospects for every one of those young people.‎”

Gibb said he was pleased to see rises elsewhere in the proportion of pupils taking more academic subjects, which appeared to be most notable in sciences, with schools moving away from entries into alternative Btec qualifications.

There were slight falls in the numbers gaining the highest A* qualification, for the fifth year in a row, while the proportion of 16-year-olds gaining A* or A fell by 0.6 percentage points.

Falls in English and maths were among the most shocking. The overall proportion of pupils getting A*-C in English plummeted 5.2% to 60.2%, and maths suffered a drop of 2.3 percentage points for grades A*-C.

The fall in the number taking GCSE English – about 200,000 lower than the number taking maths – appears to have been the result of more schools entering pupils for the iGCSE English exam, an alternative qualification not included in these figures.

Figures published by exam regulator Ofqual, combining the results in both GCSEs and iGCSEs, revealed that the net effect was grades only slightly lower than previous years.

There was a small increase in the proportion of A* grades in English, up 0.2 percentage points to 3.3%, but again A*-A was down by 0.9% to 13%. Maths at A* was also down 0.4 percentage points, and 0.6% for grades A*-A.

There were significant drops in the A*-C proportion for several subjects, including computing (down 4.7 percentage points), science (-3.8), history (-3), geography (-2.8) and maths (-2.3).

“There is a significant movement in this year’s entries, which impacts on results and creates a very complex national picture,” said Michael Turner, the director of the Joint Council for Qualifications, which represents the combined examination boards.

“We see shifts not only between subjects but across qualifications and year groups. This is driven by several factors, including performance measures and resit policies in England.”

The gender gap in exam results increased slightly, by 0.5%, with 71.3% of girls’ entries awarded at least a C grade compared with 62.4% of boys’.

Girls also outperformed boys in achieving the very top grades, with 7.9% getting an A* compared with 5%, although both saw a slight decline.

In England alone the A*-C pass rate dropped from 68.8% in 2015 to 66.6%.

Northern Ireland – where education is dominated by grammar schools, the subject of debate in England – bucked the national trend with a rise in A*s and As as well as a rise in the headline pass rate to 79.1%.

Wales managed to hold steady with a pass rate of 66.6% – which will come as some relief after last week’s disastrous A-level results.

The abrupt falls in England come as a surprise, because grades for year 11 pupils are set according to a process known as comparable outcomes, which links them to the performance recorded by the same cohort of pupils when they sat key stage 2 tests five years earlier.

The use of comparable outcomes has ended the charges of grade inflation that accompanied the increase in pass rates since 2000. The proportion of GCSE grades awarded has fallen at all levels from their peaks in 2011.

For the first time schools will now be rated according to a new performance measure, known as Progress 8, which charts how well pupils performed compared to their peers nationally based on their key stage 2 exams. It replaces the government’s previous measure, the proportion of pupils achieving C or higher in five GCSE subjects including English and maths.

School-level results under the new Progress 8 measure will not be available until the Department for Education releases its official calculations later this year. It will be published alongside another metric, Attainment 8.

This year’s results mark the last of the current format of GSCEs for maths and English, which will be replaced next year by exams graded on a 9-1 scale, with more rigorous content. Other subjects will be replaced over the following two years.

Kevin Courtney, the general secretary of the National Union of Teachers, said there were some troubling trends in the results, in particular the drop in entries for creative arts subjects as schools conformed to the government’s “prescriptive” English baccalaureate (Ebacc) requirements – a school performance measure.

“Teachers were faced this year with the introduction of less accessible syllabuses, including cuts to course work and no oral element in English,” he said.

“Similar practices are now being extended to other subjects and will reduce the opportunities for students to show what they can do.”

Among the independent schools celebrating the results of their pupils was King’s College School in Wimbledon, south-west London, where three students each achieved 14 A*s. Overall, 96% of the £20,000-a-year school’s entries gained A*s or As, slightly fewer than in 2015.