The GCSE A* to C pass rate in Wales has fallen to its lowest level since 2006, after some of the biggest changes in decades to the exams system.
The exams regulator said an increase in 15-year-olds sitting exams early was mainly to blame.
The A* to C pass rate fell to 62.8% after it had remained stable at 66.6% for three years.
The percentage of the highest A* to A grades also dipped to 17.9%, down from 19.4% in 2016.
Qualifications Wales had warned that an increase in early entries in some subjects was likely to mean lower results.
The Joint Council for Qualifications (JCQ), which collates exam data for Wales, England and Northern Ireland, said changes in entry patterns – particularly for 15-year-olds – and the high proportion of pupils who took the two new mathematics GCSEs early in November, had a substantial impact on the results.
The overall A* to G pass rate was also down from 98.7% to 96.9%, but the percentage of the highest A* grades remained at 6.1%.
While the fall in grades has been linked to the increase in 15-year-olds sitting exams early, the performance of 16-year-olds also fell by 2.8% for A* to C grades.
This summer’s 16-year-olds were the first to complete courses in six reformed GCSE examinations.
But Qualifications Wales said their performance in these exams had either improved or remained stable.
While Welsh pupils were still graded A* to G, students in England received new numerical grades for some subjects.
The new GCSEs are in English language, English literature, Welsh language, Welsh literature, mathematics: numeracy and mathematics, and the regulator said it was confident standards had been maintained.
Mathematics: numeracy was first examined in November and, by collating data for the two mathematics GCSEs from November and this summer, the A* to C pass rate for 16-year-olds was 60% in GCSE mathematics and 58.5% in mathematics: numeracy.
The A* to C pass rate for GCSE mathematics was down on the previous year’s figure of 65.6%.
In English language, the JCQ said the lower overall results for English language were explained by the lower outcomes of the large number of 15-year-olds entered for the exam.
About 65% of all Year 10 students were entered for the exam this summer, but the results for 16-year-olds saw an improvement according to the exams board.
There was also an increase in early entries for GCSE Welsh language and results have remained stable for 16-year-olds. Welsh literature saw a small improvement in performance but a substantial fall in entries.
There was also a substantial 44% fall in entries for English literature, but an improvement in results.
The fall in the number of students taking modern languages continued, with a 10.9% drop in entries for French and a 30.8% reduction in German, while Spanish saw a small increase.
The JCQ said French results were considerably lower at the top grades, while German results were up.
It also highlighted a trend towards reduced entries in a range of “optional” subjects.
Meanwhile, girls continued to outperform boys with 67.2% of girls’ grades at A* to C compared with 58.2% for boys.
The gap grew this year for the A* grade, with an increase in the percentage of the highest grade for girls to 7.5% while the figure for boys was 4.7%.
Emyr George, the director in charge of General Qualifications, said there has been “a notable increase” in the number of students sitting their exams early at the end of Year 10 as opposed to the end of Year 11.
He said there had been an increase of about 40% in Year 10s awarded grades this summer.
There were 334,100 entries for the exams, up from 303,620 last summer.
Qualifications Wales said the increase in exam entries was mainly driven by the extra maths GCSE and the increase in Year 10 entries in some subjects.
It appears to be the main reason why the overall number of entries for English language this summer is about 24,000 higher than last year at 59,050.
In England, English and maths will be graded numerically this year for the first time from nine at the top end of the scale down to one.
The changes make it more difficult to compare the overall performance of pupils in different parts of the UK.
Scotland has a separate system while Northern Ireland is also keeping the A* to G grading, although some pupils have been taking numerically graded exams.
The regulator says the qualifications remain broadly equivalent across the nations.
Mr George said: “It’s quite a significant year and one we’ve been preparing for to ensure that those students taking the new qualifications in Wales this year can be confident that they’ve not been unfairly disadvantaged in any way by being the first to tackle those qualifications.”