The BBC's Sue Littlemore: "For many, the claim that A levels have become harder is hard to believe"

James Sabben-Clare, headmaster of Winchester College, said the government and exam boards should admit that the value of the A level as the "gold standard" had been debased.

Speaking to fellow independent school heads at the opening of the Headmasters' Conference in Bristol on Monday, he said: "Only 20 years ago, the most successful schools had about a third of their candidates getting A grades - this year it was three-quarters."

Winchester College this year came out top in an A level league table for independent schools.

But he said: "I know that the leavers of 1999 were no cleverer overall than those of 20 years before."

Higher expectations

Mr Sabben-Clare, who is Chairman of the Headmasters' Conference, said A-levels had become available to an ever-widening section of the school population, which was a good thing.

But they could no longer do justice to the country's brightest pupils, which was something the government could not ignore.

Is the A level exam testing enough?

"A system that does not identify scholars is not likely to be nurturing them either," he said. "And that is something we must not be complacent about. For our abler pupils we need to set the expectations higher."

Mr Sabben-Clare said that for all the government's commitment to raising standards for the many, it was important not to forget the few - the most able.

"The few, in the higher branches of the academic tree, will be essential to the future of the country in an age when intellectual power will be man's most valuable commodity.

"Serving the interests of the most able is not a matter of handing out more A grades or devising yet more exams."

He dismissed a study by the Qualifications and Curriculum Authority - often referred to by ministers - which said A level standards had been "broadly maintained".

"The content has in most subjects, probably all, been reduced. The methods of marking are also changing. There has been a great deal of fuss about over-rigid mark schemes in the humanities, which prescribe set forms of answer and penalise the imaginative and unorthodox response."

Mr Sabben-Clare later denied that A levels were getting easier, but said they were "easier to do well in".

'GCSE exam is a hindrance'

He added that the development of AS levels, and the growing numbers continuing school after age 16, made it even harder to justify the GCSE exam.

"You don't need to do a GCSE if you are going to study an AS level in the same subject just a year later. That doesn't mean abolition. But for the abler candidates, the present exam is more of a hindrance than an aid to progress."

Ministers have acknowledged the need to stretch the most able students at A level, promising to develop new "world-class tests" in a number of subjects.

These extension papers, which would test the existing A level syllabus in greater depth, are due to be piloted next year, and introduced nationally in 2002.

'Rigorous standards'

But Mr Sabben-Clare dismissed the plan. "It depends what they're going to be, if they're going to happen at all," he said.

"Even at best, they could only provide part of the answer. It is the culture of continuous external testing from ages 14 to 18 that is damaging."

Responding to Mr Sabben-Clare's comments, Education Minister Malcolm Wicks said the government recognised the need to stretch the most able students.

"That is why we are introducing world-class tests at A level in a range of subjects," he said.

"Several universities, including Cambridge, have already expressed interest in using the new awards in their selection procedures.

"Revised A level syllabuses being introduced next year will maintain rigorous standards."

The changes will include new AS levels, which the government hopes pupils will take in up to five subjects in their first year in the sixth form.