The government's exams advisers have told it that its plans for two new maths GCSEs are not going to work.
Maths GCSE covers a broad range of aptitudes
The government wants one GCSE that all teenagers must sit, which covers all the basics but also prepares them for A-level maths courses.
It wants a second "further maths" qualification for higher achievers.
But the QCA watchdog says making the first GCSE self-contained will be "challenging", and taking both will be the best preparation for A-level.
The QCA (Qualifications and Curriculum Authority) first set out various options a year ago - rejecting the recommendation of an expert review of maths teaching that there should be a simple double award.
It advocated instead a double award "on the basis of two single awards" which could either be taken together or one after the other.
In reply, Schools Minister Lord Adonis said he and his ministerial colleagues wanted GCSE 1 and GCSE 2 to be completely separate.
The first should be a "gatekeeper" qualification, incorporating the sort of functional numeracy skills needed for everyday life, but also continuing "to support progression to study maths at A-level and beyond".
The second should be "a further maths qualification, aimed at both higher achievers and more motivated students".
Now QCA chief executive Ken Boston has said: "The remit to develop a second GCSE in mathematics states that the first GCSE, including functional mathematics, must be sufficient to ensure progression to A-level.
"This will be challenging."
He adds that the focus on problem-solving and more open-ended questions in GCSE 2 means it "will doubtless be seen as the natural stepping-stone to A-level mathematics".
"It seems right, therefore, to acknowledge that achieving both GCSE 1 and GCSE 2 will provide the best preparation for success at A-level."
Dr Boston said young people's mathematical aptitude was very broad.
"While there are some 14-year-olds still struggling with basic arithmetic, we have all come across those few others who are genuinely pushing at the frontiers of advanced mathematics, and clearly destined for brilliant careers."
A spokesperson for the Department for Education and Skills said children who studied either maths GCSE would get the same full national curriculum programme of study as at present, "testing the full ability range and enabling them to take A-level".
"But, in addition, the optional GCSE will enable pupils to further broaden their interest in the subject.
"We have asked the QCA to pilot the second GCSE between this September and its introduction in 2010 which will help us refine its structure."
Last month the Advisory Committee on Mathematics Education (Acme), which has been consulted about the new exams, registered its own concerns that GCSE1 would not provide the proper foundation for further study at A-level.
Spokesperson Margaret Brown said ministers should now tell schools that most pupils should take both exams.
"It is regrettable that the government has so far failed to give a strong signal to schools, teachers and parents that the two maths GCSEs when introduced will be the best preparation for further study of the subject at A-level and beyond.
"If this is not the case, we risk young people being under-prepared for the rigours of studying maths at A-level - a concern that would appear to be shared by QCA, the government's own advisers on the school curriculum," Prof Brown said.