The Government's flagship new diplomas are at risk of failing unless ministers scrap A-levels and GCSEs as free-standing qualifications, headteachers have warned.
Ministers want diplomas, which combine practical skills with theory, to compete with A-levels to see which courses are the most popular in schools.
But the Association of School and College Leaders said teenagers and their parents would be reluctant to make "a leap of faith" and sign up for diplomas that could prove a failure.
ASCL general secretary John Dunford said schools needed to know whether A-levels would survive or be scrapped in favour of diplomas, a decision the Government is not scheduled to take until 2013.
He called on ministers to bring A-levels and GCSEs within diplomas to make the increasingly complex exams system simpler to understand.
Speaking at the union's annual conference in Brighton, Dr Dunford said: "The fear is that the take-up will not be as high as we would like it to be and certainly not as high as the Government would like it to be. People aren't going to go for something they don't understand."
The union called on ministers to bring existing A-levels and GCSEs within the diploma system, so all pupils would study for a "general diploma".
This would spell the end of GCSEs and A-levels as free-standing qualifications. The first five diplomas will be taught in England from September in construction, engineering, IT, health and creative and media.
The Government estimates that about 40,000 teenagers will take the diplomas in the first phase of the system this year. But schools are already experiencing problems recruiting pupils for the courses, according to ASCL members.
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Mr Dunford said: "One of the risks is that the take-up this September for diplomas in some institutions is unviable and therefore those courses won't run."
