Proposed new maths GCSEs could leave students unprepared for A-level study, experts on maths teaching have said.
Maths GCSEs are to change by 2010
The subject is being revised in England to meet a ministerial demand for "functional" numeracy in the main GCSE.
The Advisory Committee on Mathematics Education (Acme) said having another for those wanting broader study risked curbing students' options at age 13.
The Department for Education and Skills said it should not be mandatory to take both GCSEs to study maths at A-level.
Speaking for the independent committee, Professor Margaret Brown said the new GCSEs could be "a very good thing"
"They should expose the majority of pupils to a greater range of types of question and provide them with both a deeper understanding of maths and greater competence in applying their skills to everyday life and employment.
"The danger in 'decoupling' the two GCSEs is that the 'compulsory GCSE', without the second GCSE, will not provide the proper foundation for further study at A-level.
"Pupils must not be misled on this issue."
Acme wants at least 60% of youngsters to be entered for both versions.
A further concern is that if schools only have to report on the compulsory maths GCSE for league tables purposes, that will be what they will focus on - especially now that having good maths (and English) GCSEs is now the key benchmark.
Acme wants incentives, at least for the first few years, to ensure all English schools and colleges offer the second GCSE.
A DfES spokesperson said the existing maths GCSE was not being divided or diminished.
"As is the case now there will continue to be a maths GCSE for all students which covers the full national curriculum programme of study, tests the full ability range and provides a solid foundation for further study."
But from 2010 there would also be a second optional maths GCSE for those with an interest in broadening and deepening their interest in the subject.
"It may well be the case that those seeking to study maths at A-level will want to take both GCSEs in future but it should not be a requirement.
"We will look closely at the structure and take-up of the second GCSE as it is piloted between this September and its introduction in 2010."
But the Council for the Mathematical Sciences endorsed the Acme statement and wants the government to go further.
Its chair, Professor Sir David Wallace, said: "The quality of mathematics studied at A-level and in higher education must be maintained in order to supply the skilled people that the country needs.
"For the economic tower that is built on the mathematics taught at GCSE not to collapse, the government must take Acme's concerns very seriously.
"All schools must offer both GCSEs, or they risk limiting their pupils' future success in many sectors."