The International GCSE is not suitable for assessing what pupils in England learn, a review by the qualifications watchdog has concluded.
The education department wants to know people's views
The IGCSEs are increasingly popular with independent schools in England.
State schools are not funded to offer them, as they are not officially accredited, but Schools Minister Lord Adonis has called for a debate.
A review for the Qualifications and Curriculum Authority (QCA) found major differences between the types of exam.
Its report, whose publication Lord Adonis had called for, involved independent consultants reviewing syllabuses, question papers and marking schemes in English, French, maths and science.
They compared the IGCSE syllabuses from the two exam boards that offer the qualification, Edexcel and Cambridge International Examinations, with two GCSE syllabuses.
It is understood that the QCA has not made any recommendation to ministers based on the report, but its attitude seems clear from its circular findings.
- in all four subjects there were major differences between the IGCSEs and the GCSE examinations
- these meant the IGCSE examinations did not meet the GCSE subject criteria - the brief the QCA gives the exam boards which design the qualifications
- those GCSE criteria are "tightly bound up" with the Key Stage 4 (age 14 to 16) programmes of study
- so the IGCSE cannot be regarded as assessing those programmes to the same extent as the GCSE does
"This probably reflects the different contexts in which the IGCSEs were developed," the report adds.
In English, for example, the treatment of speaking and listening varied and there was significantly less prescribed reading in the IGCSEs.
In French, the Edexcel IGCSE did not include a speaking test.
In maths, neither IGCSE had a non-calculator paper. In GCSE one of the two papers at each tier did not permit calculators.
When he called for the report's publication, Lord Adonis said the government could then invite a wider debate within the education community on the IGCSE's use in the state sector.
"We will then look at the outcomes of that debate with an open mind," he added.
A spokesman for his department said it had made clear that it was sticking with GCSEs and was happy with the curriculum, so anything new would have to fit in with that.
Shadow schools minister Nick Gibb repeated his call for the IGCSE's availability in the state sector.
If the government and the QCA refused this, he said, an even greater divide between the state and independent sectors would emerge.
"Many of the criticisms of the IGCSE in the QCA report are not about the merits of the exam itself but are confined to whether or not the exam is consistent with other QCA requirements.
"This should not be a reason for refusing state schools this important new freedom."
Europe's largest assessment agency, Cambridge Assessment, urged educators to look beyond the QCA comparability study.
The report itself had said "probably the most significant limitation of the study derives from the fact that it did not consider student work", it noted.
This had "implications in terms of the confidence that can be placed on the judgements about demand".
The group director of assessment research and development, Tim Oates, said: "Without seeing candidates' scripts and mark distributions, it is difficult to determine the demand of the qualifications.
"It also failed to look at the IGCSE in a wider international perspective."
He said it would be unfair for state schools not to be able to offer the qualification.