More than one in four UK A-level entries were awarded the top A grade this year, results show.
Two years' work leads up to the moment the results are opened
Figures from the Joint Council for Qualifications showed 25.3% of entries were graded A, up from 24.1% last year.
The improvement in A grades in independent and grammar schools over the past five years has been double that in state comprehensives.
The national pass rate rose for the 25th year in a row, with 96.9% graded A to E, up from 96.6% last year.
Girls continued to outperform boys in every major subject except for modern foreign languages and further maths, though the gender gap again narrowed very slightly.
The statistics relate to more than 800,000 A-level entries and more than 1.1 million AS-levels.
About 310,000 candidates have been getting their individual results, mostly in England, Wales and Northern Ireland, some 50,000 of them online.
Looked at over the five years since the Curriculum 2000 changes to the form of A-levels were made, the proportion of entries from independent and selective schools awarded an A has increased by about six percentage points.
This was double the rate of improvement in the majority of state schools.
The difference was revealed by the director general of the biggest board, AQA, Mike Cresswell, who dismissed suggestions that exams were getting easier.
He said the improvements were down to improved teaching and learning.
Dr Cresswell added: "Whatever the usual grumpy old people want to say about how it was much harder in their day we want to say the students have done very well, they are a success story and we should be proud of them."
And he reminded people that less than a tenth of candidates achieved three A grades, representing just 3% of 18-year-olds.
As usual, entries from Northern Ireland easily outperformed the rest of the UK, with a third (33.2%) getting A grades and a pass rate of 98%.
In Wales, 24.1% of entries were awarded As while 97.1% passed.
Of the English entries, 96.9% passed and 25% achieved As.
The top two subjects in terms of popularity were English and maths, with maths continuing its upward trend. Entries in maths were up 7.3% this year while those in further maths rose 8.3%.
There was also a small rise in the numbers of people taking chemistry and physics but a slight fall in biology.
Pass rate / Grade A
UK: 96.9% / 25.3%
England: 96.9% / 25%
Wales: 97.1 / 24.1%
NI: 98% / 33.2%
Entries in modern foreign languages were largely stable. There was an increase in entries for German and Spanish but a decrease in those for French.
There were marked differences between subjects in the grades awarded: with 43.7% of maths A-levels given an A grade compared with 14% of those in media studies.
"What that tells you is that there are differences in how good those candidates are at those subjects," Dr Cresswell said - no-one would suggest that maths was somehow easier.
The biggest percentage increase in entries, from a small base, was in critical thinking.
The chief executive of the OCR exam board, Greg Watson, said it provided the sorts of skills in weighing information and evaluating arguments that were sought by both universities and employers.
The Liberal Democrats called for an independent review of A-levels but this was ruled out by the government.
Schools Minister Jim Knight said one had been carried out three years ago and it was a "real shame" the annual debate about standards undermined pupils' performances.
He said: "I warmly congratulate students and teachers on these excellent results.
"Sustained progress in A-level results over the last decade is down to high quality teaching and strong investment in our schools."