The headteacher of the school with the best GCSE results in the country warned yesterday that the exam was "in crisis".
Martin Stephen, of St Paul's Boys' School in Barnes, London, which has fees of up to £20,895 a year, was speaking as it emerged that 250 of the best-known independent schools had ditched the exam in favour of the international GCSE.
The figure amounts to a boycott of the main exam by half the independent sector because heads do not believe it stretches pupils.
Mr Stephen, whose students achieved an average point score of 600.3, equivalent to 10 A* grades per pupil, said the move away from the national exam was "a stampede". Last year 15 schools switched.
The move is in danger of creating a two-tier examination system with the IGCSE being the main vehicle for the independent sector and national GCSEs for state schools.
Figures for independent schools yesterday showed that 57.2 per cent of all candidates achieved an A* or A grade this year, with 93.5 per cent of pupils achieving five A* to C grade passes.
But Mr Stephen said that GCSEs were not sufficient preparation for A-levels. "The trouble is it falls between two stools," he said. "It should be either a guarantee of a minimum set of skills for those who leave education at 16 or preparation for moving on to the post-16 stage. It is neither."
Schools that take the IGCSE can opt to exclude coursework - which has been criticised for encouraging cheating - and rely on an end-of-course exam.
Yesterday, leaders of the country's independent schools, backed by the shadow Education Secretary David Willetts, demanded that the Government recognise the international exam. At present, it is neither validated by the Qualifications and Curriculum Authority, the Government's exams watchdog, nor included in league tables.
The former precludes state schools from offering it because they will not get funding to enter pupils, although some of Tony Blair's new flagship academies have expressed an interest.
Edward Gould, chairman of the Independent Schools Council - which represents the majority of private schools - said: "It remains illogical that IGCSEs are not yet recognised."
Mr Willetts added: "In his Budget speech, Gordon Brown pledged to put state schools on an equal footing with independent schools. So why on earth is the Government treating maintained schools so unfairly in preventing them from offering the IGCSE? The Government counts a child who gets an A* in an IGCSE as a fail."
He said schools should be free to offer a range of choices - including the IGCSE instead of GCSEs and the International Baccalaureate and Pre-U, a traditional alternative to A-levels.
A spokeswoman for the QCA said it had not received an approach from exam boards to validate the IGCSE. But the Oxford and Cambridge and Royal Society of Arts board said there had been no approach because it was known the answer would be "no" unless the name was changed or its content brought more into line with GCSEs.
The OCR has written to Alan Johnson, the Education Secretary, asking that the IGCSE results should be included in government exam league tables.
A spokesman for the Department for Education and Skills said the QCA was carrying out a study to see how the IGCSE compared to the national exam.
Yesterday's independent schools results also showed that - despite St Paul's topping the table - girls were outperforming boys, taking six of the top 10 places.Reuse content