More than one in four A-levels are expected to be awarded at least an A grade this year, but pass rates are set to stall, it was predicted on Sunday.
In a bid to avoid grade inflation, the exams regulator Ofqual has told exam boards they will be asked to justify results that differ wildly from previous years.
It means that the proportions of students awarded top grades are unlikely to increase greatly from 2011 levels.
Professor Alan Smithers, director of the Centre for Education and Employment at Buckingham University said he would expect to see the proportion of exams awarded an A* rise only slightly, from 8.2% in 2011 to around 8.3%.
"If Ofqual had not introduced 'the principle of comparable outcomes' to nail inflation, I would have expected the percentage of A* to have gone up by quite a lot, say to 8.5% or more," he said.
The A* has now been awarded for three years, schools are used to preparing pupils to achieve it, and universities are asking candidates for the grade, he added.
But Ofqual has said it wants to see a distribution of grades similar to last year, which were tied to 2010 A-level results, Prof Smithers said.
Last summer 27% of A-levels were awarded an A* or A, the same as in 2010, It was the first time in more than a decade that the results had remained static.
"This year, it could go either way, but I am guessing that Ofqual will hold the percentage of A*-A close to 27%, but because there are good reasons for it happening allow the percentage of A* to increase say to 8.3%," Prof Smithers said.
"The interesting thing now is we know that what Ofqual is intending is that the pattern of results this year should be more or less the same as last year. Other things being equal, they'll be asking the exam boards stiff questions if there's a big movement from the results last year."
The move is likely to intensify the scramble for university places. Surveys suggest that universities will have little room for students who miss their grades this summer.
Prof Smithers predicted that overall pass-rates will not rise dramatically, with "just over a quarter getting A*-A, half getting A*-B and three quarters A*-C".
In 2011, 52.6% of A-level exams achieved at least a B, and 76.2% got at least a C grade.
Prof Smithers said he thought it was unlikely that pass-rates will drop at A-level because there have not been any major changes to the exams.
"The kinds of things that were driving up performance are still there, for example, schools are judged on the exam performance of their pupils."
He said it was "just possible" that boys could do better than girls at A* this summer, after closing the gap last year.
"I think most A*s go in subjects like maths and further maths and physics and German. And certainly as far as sciences are concerns, the boys tend to do better in those. So there's proportionately more boys in those subjects where A*s are awarded."
"The fact that boys tend to opt for maths, further maths, physics and so on means they're in a better position to get more A* than girls," Prof Smithers said.
But he added that the current A-level system, which allows exams to be taken throughout the course, does tend to suit girls more than boys.
An Ofqual spokesman said: "Our job is to make sure that grades are right, and that what is required for each grade stays steady.
"Where we see differences year on year, or between one exam board and another, it is our job to challenge that and to see whether there is any evidence or explanation for it.
"Results do go up or down, for various reasons. They don't stay exactly the same each year, in each subject. But we have to be as sure as we can be that any movement is for a good reason, and that is what we do.
"We don't 'fix' grades - but we do make sure that grades are right. We have developed our approach with the help of the best experts in the field, and we are open about what we do."
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