The number of students awarded an A-grade or A* has fallen to the lowest level since 2007, national figures show.
The figures, published by the Joint Council for Qualifications (JCQ), cover A-level entries from students in England, Wales and Northern Ireland.
Around 300,000 students are receiving their results today.
In total, 25.5% of UK entries were awarded an A or A* grade this summer, the lowest proportion since 2007 when it was 25.3%.
Girls narrowly clawed back the lead from boys in terms of A* and A grades, although boys still outperformed their female classmates in the highest result.
For the first time, there were more entries for A-level science subjects from girls than from boys.
The shift comes after a major push in recent years to encourage girls to study science and maths-based subjects.
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Spanish has become the most popular foreign language at A-level, overtaking French for the first time.
A* grades were introduced in 2010. Before this, the highest result was an A.
Girls are back in front in terms of top grades, with 25.5% of entries handed at least an A, compared with 25.4% of boys - a gap of 0.1 percentage points.
But on A* grades alone, boys performed better, with 8.2% of entries getting the highest result, compared with 7.5% of girls' entries.
Girls are now more likely in general to take a science A-level than boys, the data shows.
For biology, chemistry and physics A-levels, there were 84,111 entries from girls this year, compared with 83,133 from boys.
The change has been driven by more girls than boys taking biology and chemistry.
But boys are still significantly more likely to take physics, with more than three times as many entries than their female counterparts (30,159 compared with 8,799).
Yesterday, leaked documents showed students scoring just over half marks in A-level maths will be rewarded with an A grade.
Grade boundaries for Edexcel's maths A-level show students who gained 165 out of a possible maximum of 300 marks (55%) will be awarded an A.
Separate documents show that those who took the OCR exam board's A-level maths qualification will walk away with an A if they achieved 54% across all papers - a total of 161 out of 300.
Last year, 184 marks (61%) were needed for an A grade in Edexcel's maths A-level, while for OCR's qualification in the subject, the required mark was 197 (66%).
Edexcel's parent company Pearson said that grade boundary information is shared with schools a day in advance to help teachers prepare and that the information was shared via a password-protected, secure website.
OCR said their grade boundaries are released to schools in advance and they share them with students on results day, a system designed "to minimise the chance of students feeling anxious" if they see the boundaries without their grades.
The leaked Edexcel document also shows that this summer, 43 marks (14%) would result in an E grade - considered a pass, while just over a third of marks (34%) would mean a C grade.
For OCR, 40 marks (13%) is the required threshold for an E grade this year, while 100 (33%) is needed to obtain a C.
These figures relate to overall grade boundaries for new specification maths A-levels.
Maths is one of the last subjects to be reformed as part of a major overhaul of exams in England.
This summer is the first time that grades for new specification A-level maths are being awarded to the vast majority of students.
Last year, just a small number of students took the reformed qualification - those who sat the exam after completing the course in just one year, rather than the usual two.
A small number of students may still be studying the old "legacy" maths A-level course.
The leaked documents also give grade boundaries for other A-level qualifications offered by Edexcel and OCR - two of the biggest exam boards in England, and there are some differences between the two.
For example, 69% (208 out of 300) in an Edexcel English Literature A-level achieves an A grade, while students taking the subject with OCR require 177 marks out of 200 (89%).
And in chemistry, the A grade threshold for Edexcel students was 202 out of 300 (67%), compared to 196 out of 270 for those taking OCR's course (73%).
For the traditional biology A-level offered by Edexcel, 175 out of 300 (58%) was required for an A grade, while for the OCR version, the A grade pass mark was 158 out of 270 (59%)
A Pearson spokeswoman said: "Per JCQ guidelines, all boards share grade boundary info with schools a day in advance to help teachers prepare and support their students better on results day.
"Our systems are working as they should and the information was shared today via a password-protected, secure website.
"Boards do ask schools not to share this widely to avoid unnecessary stress for students awaiting their results.
"Schools are trusted to treat the info confidentially on behalf of their students and the vast majority do."
An OCR spokesman said: "We provide schools and colleges with results information including grade boundaries the day before results day on a special site which is accessible to exams officers.
"Grade boundaries are then released by schools to their students on results day and we publish them on our website. We do it this way to minimise the chance of students feeling anxious if they see grade boundaries without their results - which can lead some to jump to the wrong conclusion.
"We are very grateful to the vast majority of exams officers and teachers who support this staggered approach."
Geoff Barton, general secretary of the Association of School and College Leaders, urged students "not to lose sleep over grade boundaries".
"We are extremely disappointed if grade boundaries have been leaked ahead of results day," he said.
"The problem is that anxious students will pore over this information trying to work out what this means for their results.
"This is a pointless exercise because grade boundaries are set to allow for differences in the difficulty of papers so that students are not disadvantaged from one year to the next.
"We would urge students against losing sleep over grade boundaries and to wait for their results tomorrow."
There were reports earlier this year that some students were complaining that one of the new Edexcel A-level maths papers was too hard.
In a video statement last week, Pearson addressed these concerns, saying it was aware that some had found Paper 2 "more difficult than they were expecting".
The board said it wanted to reassure students that independent experts had analysed the paper and confirmed it was a "fair and valid exam testing across the ability range and the course curriculum".
This summer, questions from one paper of Edexcel's new A-level maths qualification were leaked ahead of students sitting the exam.
A police investigation is ongoing.
Meanwhile, almost a quarter of a million students have opted to take a vocational qualification this year, figures show.
Early data from exam board Pearson suggests that BTec Nationals - equivalent to A-levels - remain popular with learners as the future of the courses hangs in the balance.
In total, 246,900 people across the UK have completed a Level 3 BTec so far this year, in subjects such as business, sport, health and social care and engineering and construction.
Pearson said a comparable figure for 2018 is not available.
The qualifications are in the process of being reformed, with the overhauled courses including external assessment.
The reformed courses "represent a raising of the bar in terms of assessment and support for learners", Pearson said
