Research by the National Council for Vocational Qualifications concludes that GNVQs "must be for all students, to avoid sheep-and-goats selection". But the evidence shows that this cannot be managed in the 20 per cent of the timetable freed up by Sir Ron Dearing, the Government's chief curriculum adviser. The Dearing report did not envisage that all pupils would choose the vocational option.
The GCSE-oriented, subject-based national curriculum timetable is still too rigid to cope with the full ability range, say the NCVQ researchers. Nor would just studying a GNVQ Part 1 (equivalent to two GCSEs) permit an adequate range of studies and work experience.
An investigation by the NCVQ of options for 14 to 18-year-olds began in 1993 - 18 months before Sir Ron announced his reforms.
There was concern within the council last autumn that the Government was seeking a quick political fix rather than waiting for the results of the research.
Those results, published this week, show that attempts to slim the curriculum were unlikely to release time for extra studies.
Greatest pressure for a more radical reform of the curriculum is coming from the Government's flagship city technology colleges - five were involved in the NCVQ research - which are increasingly adopting assessment practices frowned upon by the Conservative Right.
Kingshurst CTC has extended the school day to ensure that all pupils could take a full GNVQ and up to 10 GCSEs. It was one of the 11 schools and CTCs studied by the researchers. They conclude that Kingshurst's GNVQ approach increases "analytical ability and articulacy for all students, including the most able".
Other schools made far-reaching reforms in teaching methods, moving away from rigid subject demarcations. None of the 11 schools in the study followed the same model. But three broad approaches emerged.
One group concentrated on the core skills such as literacy and numeracy. Another group integrated GCSE and GNVQ studies. A third had blocks of options leading to an intermediate GNVQ award.
The research is a considerable fillip for those committed to the GNVQ and its units of study which slower learners can repeat and high achievers can take at their own pace. It also says, however, that the initiative is doomed without a commitment from the school's senior management.
Gilbert Jessup, deputy chief executive of the NCVQ, says: "The single biggest influence on the GNVQ for 14 to 16-year-olds is the impact of senior management."
Schools may be deterred from running vocational courses by the Government's demand for league tables. Differences in GCSE and GNVQ assessment styles mean that pupils who achieve fewer than the six units needed for the GCSE-equivalent award will not count in league tables.
Many schools under pressure from parents and governors to make a showing in the league tables were likely to dissuade pupils from taking the vocational route since a low-level GCSE will count for more.