Nine out of 10 elite private schools have dropped GCSEs in at least one subject in favour of harder O-level-style exams.
The Headmasters' and Headmistresses' Conference found growing support among its 250 member schools, which include Eton, Harrow and Winchester, for the International GCSE (IGCSE).
This is a qualification which is seen as harder than GCSEs, includes no coursework and has been likened to traditional O-levels.
But ministers have refused to accredit it for use in state schools, even though they have approved the use of the International Baccalaureate diploma.
Andrew Grant, head of St Albans School, said private schools were increasingly inclined to "mix and match" qualifications to suit their pupils.
He said: "What a pity that the Government could not be persuaded to be consistent in following the logic of including the International Baccalaureate in the National Qualifications Framework, by including IGCSE, too."
The survey of 100 HMC member schools found that 90% had opted for IGCSEs in at least one subject.
This was a sharp rise from last year when only 57% of respondents said they offered the IGCSE. Two years ago the figure was 35%.
More than one in 10 HMC schools (12%) are now offering the International Baccalaureate.
Only 2% said they were likely to offer any of the Government's flagship new 14-19 diplomas, which are designed to combine work experience with theory.
