Ministers have been urged to allow state schools in England to ditch GCSEs in favour of more stretching courses.
IGCSEs are not an option for state school pupils
Independent school leaders said it was "illogical" that government rules do not recognise the International GCSE - which many believe is more difficult.
The Tories added that ministers should back schools teaching the International Baccalaureate and a planned new sixth- form course, called the Pre U.
The government said IGCSEs were primarily aimed at overseas candidates.
The calls come as new league tables showed more than a quarter of private school GCSE exam entries were awarded the top A* grade this year.
The national average was for 6.3% of entries to get an A*.
Figures from the Independent Schools Council showed nearly six out of 10 (57.2%) of entries were awarded an A or better, compared with 19.1% of schools nationally.
State schools are not able to enter their pupils for IGCSEs as they only receive funding for approved exams - and these have not been accredited by the regulator, the Qualifications and Curriculum Authority.
The qualifications are not taken into account in the school league tables.
ISC chairman Edward Gould said: "ISC schools are delivering a rounded education, and in the process they continue to ensure the future supply of mathematicians, scientists and linguists.
"It remains illogical that IGCSEs, which are included in these results, are not yet recognised in the national qualifications framework especially as some maintained schools and academies would welcome the availability of this qualification for their pupils."
Shadow education secretary David Willetts said: "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 independent sector is demonstrating how popular and valuable the IGCSE is becoming, but state-school children are missing out because of what is tantamount to government discrimination.
"Moreover, schools which teach the IGCSE are actually penalised since the government tables do not recognise the qualification."
He added that the Conservatives believed that state schools should also be able to offer the International Baccalaureate and the Pre-U exam.
The Department for Education and Skills said the Qualifications and Curriculum Authority (QCA) had not received any requests to accredit IGCSEs.
A spokesman added: "The QCA are currently working with awarding bodies to see how the IGCSE compares in structure and content to GCSE."
He said that, unlike their traditional counterpart, IGCSEs are designed primarily as a qualification for overseas candidates and as such have not been designed alongside the national curriculum studied by 14 to 16-year-olds.
He added that the International Baccalaureate is already an approved qualification and taught in a number of maintained schools.