Updated: Aug 17, 2020
More than a third of results in England were downgraded by one grade from the mark issued by teachers.
Thirty six per cent of entries had a lower grade than teachers predicted and 3% were down two grades in results, after exams were cancelled by the pandemic.
The proportion of private-school students receiving A and A* was more than twice as high as the proportion of students at comprehensive schools, highlighting the extent of inequality in the education system.
Prime Minister Boris Johnson said the results were a "robust set of grades".
Labour leader Sir Keir Starmer said: "Something has obviously gone horribly wrong with this year's exam results.
"Nearly 40% of young people have had their grades marked down and that's thousands of young people whose opportunities could have been dashed."
The overall results, across England, Northern Ireland and Wales, show record highs for A* and A grades.
Geoff Barton, leader of the ASCL head teachers' union said: "While there had been an overall increase in top grades, there was a "great deal of volatility among the results at school and student level."
Students affected by the mass downgrading of A-level grades in England have been urged to join a possible legal action against the Department for Education and the exams regulator, Ofqual.
Ofqual has said any statistical model, however good, is bound to produce anomalies, and wherever these occur, schools and pupils should appeal.
Prime Minister Boris Johnson said pupils would be able to sit exams if they feel they could have done better.
"Where pupils are disappointed, where they feel that they could have done better, where they feel that there's an injustice been done to them, there is the possibility of appeal and they can re-sit, they can take a re-sit this autumn as well."
Education Secretary Gavin Williamson said the "majority of young people will have received a calculated grade today that enables them to progress to the destination they deserve, with the added safety net of being able to appeal on the basis of their mock results, as well as the chance of sitting autumn exams".
More students have been accepted on to UK degree courses this year, UCAS figures show. A total of 415,600 people, from the UK and overseas, have had places confirmed.