The pass rate in the examinations, taken by more than 265,000 students, will reach 96.2 per cent, up just 0.2 of a percentage point from 2004.

Sources close to the Joint Council for Qualifications, the umbrella group representing exam boards, told The Times that the proportion of A grades had risen more sharply, by 0.4 of a percentage point to just under 22.8 per cent of entries — the lowest since 2000.

The increase in the overall pass rate is the smallest since 1982, when exam boards lifted the limit on the proportion of students allowed to pass. It has been equalled only once before, in 1998.

This is the 23rd successive year in which the pass rate has improved. Critics of the relentless rise in A-level results will argue that a slowdown was inevitable as the pass rate neared 100 per cent.

But ministers and exam boards will maintain that the academic “gold standard” is being maintained in the face of allegations of “dumbing down”.

The results will still see a record number of students securing places at their chosen universities today, the Universities and Colleges Admissions Service confirmed.

There was confusion last night over the impact of the results on the clearing system in the final year before the increase in university tuition fees to £3,000-a-year.

Some universities gave warning of an intense scramble for fewer vacancies as students sought to beat the fee increase. But others predicted less pressure because more candidates had gained entry to their preferred university and more had opted to take a gap year.

Lord Adonis of Camden Town, the Schools Minister, made a strong defence of A levels yesterday, insisting that better results were the product of improved teaching and increased government investment in education.

He dismissed the “bogus argument” that exams were getting easier and said that students could have full confidence that standards were being maintained.

“Continued progress in exam performance is real — it is not the result of dumbing down of standards — and the roots of this success lie in a fundamental shift in the quality of teaching in our schools,” he said in a speech at a summer school for gifted children in Canterbury.

But head teachers’ leaders cast doubt on the future of A levels and repeated calls for ministers to support a baccalaureate-style diploma. In February, Ruth Kelly, the Education Secretary, rejected a diploma proposed by Sir Mike Tomlinson, the former head of Ofsted.

David Hart, general secretary of the National Association of Head Teachers, said: “I am absolutely certain that an increase in A grades and in the overall pass rate is a tremendous tribute to the work of students and their teachers. But at some stage — sooner rather than later — the Government has got to face the fact that the current system is creaking.

“Universities and employers are finding it more and more difficult to make sense of the grades for university entrance and employment purposes.”

Independent schools said that the A level was in “terminal decline” and hinted at establishing their own alternative qualification. Geoff Lucas, general secretary of the Headmasters’ and Headmistresses’ Conference of 314 leading schools, said: “It is not just that A level no longer discriminates between candidates. It no longer prepares them properly in key subjects because it has become such a mechanical exam.”

An indication of the challenge facing universities emerged at Magdalen College School, Oxford, where 51 out of 73 boys have achieved at least three A grades each.

Lord Adonis insisted that the Government would not replace A levels. “I have made it very clear that A levels are here to stay and we are not in the business of talking about any other system,” he said.

The Institute of Directors said that there was little evidence that A-level standards had fallen. Miles Templeman, its Director-General, said that employers were more worried about low levels of literacy and numeracy among school-leavers.

“There is no case for replacing GCSEs and A levels with a diploma. A revolution in the examination system would not in itself deliver the improvements that are so desperately needed,” he said.

Lord Adonis said that improved school standards were a national necessity, arguing that rising levels of academic achievement were an international phenomenon. Even now, only 14 out of 30 pupils in an average English primary school class were likely to make it through to A level, and only one would achieve three A grades.

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