The proportion of A-levels scoring at least an A grade has fallen for the first time in more than 20 years, official figures have showed.
In total, 26.6% of the exams were given an A or A*, down from 27% in 2011 - a record drop of 0.4%.
It is believed to be the biggest fall in the history of A-levels. The last time it dropped was between 1990 and 1991 when it decreased to 11.9% from 12%.
The new figures, published by the Joint Council for Qualifications (JCQ), also show that fewer A-level exams achieved an A* this year. Around one in 12 (7.9%) exams were given the top grade, which is awarded for the third time this summer, down from 8.2% in 2011.
Boys overtook girls at A* grade for the first time, with 8% of boys' entries attaining the top mark, compared to 7.9% of girls'.
The statistics also show that the overall A* to E pass rate has risen for the 30th year in a row. Some 98% of exams achieved at least an E, compared to 97.8% last year.
While many of England, Wales and Northern Ireland's 335,000 students will be celebrating their A-level results, others still face a scramble to secure a university place. Initial figures from UCAS show almost a 7% drop in the numbers of students who have already had their places confirmed.
Michael Turner, director of the JCQ, which represents exam boards, said: "Today is about celebrating the successes of our young people and recognising the hard work that has gone in to achieving these results. They, and their teachers, can be proud of their achievements."
Ziggy Liaquat, managing director of exam board Edexcel, insisted there was a "very strong set of results" and said that students "should be very proud". He added that the fall in A-grades was "marginal" and said: "The standard needed to reach an A-grade has not changed."
Brian Lightman, general secretary of the Association of School and College Leaders, said the drop in A grades was a "minor fluctuation". He said: "The important point here is, if you look at the trend of improvements in schools, the numbers now succeeding at GCSE and accessing A-levels has been a success story of our education system."