The Association for Colleges has called for the Government to commission authoritative research to determine why completion rates for the two-year-old advanced level General National Vocational Qualification are so low.
Martin Cross, chairman of the Joint Council of National Vocational Awarding Bodies, has also urged the Department of Education and Employment to investigate low completion rates at GNVQ foundation levels.
The calls for a thorough investigation follows this week's advanced GNVQ results, which show that 44,781 students completed advanced-level courses with full awards, compared with 16,500 last year.
They follow criticism from Professor Alan Smithers, a specialist in post-16 education and training, who described the completion rates as appalling. Of the 260,000 people who have signed up for the exam since it was introduced in 1992, only 103,000 have completed courses.
Further criticism comes from Dr Andy Green, senior lecturer in vocational education at the University of London Institute of Education, who claims competition for Government funding is forcing colleges and sixth forms to accept students on advanced GNVQ courses who are not qualified.
He cites Further Education Unit evidence which found that as many as 49 per cent of students were being accepted for advanced GNVQ courses with fewer than four GCSE grades A-C, recognised as a minimum requirement by most colleges.
In a report on the implications of raising Government education and training targets, commissioned by the National Council for Education and Training Targets, published earlier this year, Dr Green identified four major reasons why students drop out of vocational courses: financial hardship; lack of grounding in basic and core skills; inflexible curriculum frameworks preventing students taking suitable combinations; and inappropriate course choice.
Dr Green said the low completion rates confirmed by this week's figures underlined the need for action to make colleges stick to minimum entry requirements.
"If we are to reduce the drop-out rate, the Government needs to force colleges to accept the norm and not to accept students for courses who are not ready for them," he said.
Concern about drop-out rates is likely to be underlined by new research by London University's Institute of Education which suggests that the proportion of 16-year-olds staying on in full-time education has fallen for the first time in a decade.
Researcher Ken Spours believes the 1 per cent fall in 1994-5, which follows an annual growth of 4 per cent since 1987 to 1993, could mean staying-on rates have peaked at 72 per cent.
Reasons for the decline are thought to include an improvement in the job market, financial hardship caused by most local authorities ending discretionary grants to FE students, and the dwindling potential for recruitment among the remaining core of traditionally low attainers.
Controversy has dogged the advanced GNVQ since its introduction in September 1992.
It proved immediately popular with schools and colleges, who have long been searching for a less academic alternative to A-levels, but it has been repeatedly criticised for not being rigorous enough.
The Government decided to introduce the exam despite warnings from its vocational education advisers that it was not ready to be introduced nationally.
Ruth Gee, chief executive of the Association for Colleges, backing the call for a Government investigation, said: "Speculation is easy. We can all do it. What we need as a matter of urgency is some hard evidence on what is happening. That presently does not exist."