The government has defended A-level examinations against claims that it is becoming easier to achieve good grades.
A-level pass rates are expected to increase this year
Education minister Lord Adonis said improved results were due to a "fundamental shift" in teaching quality and were a "cause for celebration".
Last year's A-level pass rate was 96% and this is widely expected to rise when some 265,000 students receive their results on Thursday.
The British Chambers of Commerce says the exams have been "dumbed down".
And the Confederation of British Industry has highlighted the falling numbers of students taking languages and sciences.
But Lord Adonis said: "In the modern world - with better teaching and steadily higher aspirations - educational success isn't like tickets for the next Test match, where there is a fixed supply of a precious commodity.
"Rather it is like the marathon, where, with ability and proper training, growing numbers can successfully complete the course in faster times.
"But nobody would suggest that this devalues the achievements of Paula Radcliffe and the other elite athletes in Helsinki last week."
Lord Adonis said it would be a "major cause of concern" if grades were not improving and that there was a "perception gap" between reality and opinion.
The last 20 years, during which A-level pass rates had increased every summer, had seen a "sustained revolt" against "deep-seated social and educational failure".
Lord Adonis admitted, however, that some changes were being made to A-levels because more pupils than ever were getting top grades and universities were finding it hard to select applicants.
The government has recently agreed to "stretch" brighter pupils with harder questions, and to give more information about grades in the modules that make up an A-level.
Liberal Democrat education spokesman Edward Davey said: "It is time to replace A-levels with new diplomas. By rejecting reform of the A-level system, the government is burying its head in the sand."
David Hart, general secretary of the National Association of Head Teachers, said constant improvements in pass rates showed "the current system is creaking".
He called for the government to adopt a broader, diploma system combining academic and vocational courses to replace A-levels and GCSEs.
Geoff Lucas, general secretary of the Headmasters' and Headmistresses' Conference, which represents many leading independent schools, said A-levels were in "terminal decline".
Grade inflation was making it impossible to distinguish between brighter candidates, he added.
Several independent school heads are promoting a move away from A-levels towards the International Baccalaureate, which, they say, has higher academic standards.
Earlier, a British Chambers of Commerce survey suggested 50.4% of small firms have trouble recruiting adequately skilled staff.