New vocational GCSEs are being undermined because teachers are poorly prepared and schools are not providing relevant work experience, inspectors said this week.
Teaching standards and pupil achievement on the courses have not reached the level of traditional GCSEs, according to an Office for Standards in Education report.
Pupils' achievement is unsatisfactory in a quarter of vocational lessons compared with one in 11 traditional courses.
The report calls for teachers to be trained in vocational subjects and for minimum standards of competence for teachers of vocational subjects.
Schools should also be given clear guidance on how to provide vocational experience, inspectors said.
Despite the findings, Ofsted said the new courses improved the behaviour of difficult pupils and allowed teenagers to pursue a more diverse curriculum after 16.
Vocational GCSEs in eight subjects, each worth the equivalent of two traditional qualifications, were taken by pupils for the first time this summer. The subjects are: applied art and design; applied business; engineering; health and social care; applied ICT; leisure and tourism; manufacturing; and applied science.
The chief inspector, David Bell, said: "The new GCSEs have got off to a satisfactory start but we need to look more carefully at how to bring to life the vocational element of the courses to ensure that pupils have more relevant work-related experiences and are able to translate theory into practice."
Inspectors found wide variations in the quality of teaching and learning between subjects. Some subjects - particularly leisure and tourism - are targeted at low-attaining pupils. This leads to the course being held in lower regard than traditional GCSEs and often results in lower standards.
Teachers are often unfamiliar with the new courses and lack relevant qualifications.
The report criticises schools for not stretching pupils enough. It says:
"In a large minority of schools the volume of work done by pupils and the breadth and depth of their studies do not always add up to the weight of a double-award GCSE, especially (but not exclusively) in schools where insufficient teaching time is allocated."
It also says rooms designed for academic studies should be adapted for vocational lessons.
Ken Boston, chief executive of the Qualifications and Curriculum Authority, said: "These qualifications have bedded down well in many schools and colleges."
John Bangs, the National Union of Teachers' head of education, said:
"Further improvements are needed, but vocational GCSEs are working and Mike Tomlinson would do well to take this into account, rather than opt for over-complex approaches at 14 to 19."
Developing new vocational pathways: final report on the introduction of new GCSEs is available at www.ofsted.gov.uk