Pupils taking new vocational exams will receive too much credit for insufficient work, according to inspectors.
Vocational GCSEs were taken for the first time this summer, but schools are not devoting enough time to the courses to justify their status as double GCSEs, the Office for Standards in Education is expected to say next week.
Ministers hope the new exams in eight subjects, including engineering, leisure and tourism, and health and social care, will raise the status of vocational education, help pupils disaffected with the traditional curriculum and encourage a mix with academic options.
But Ofsted's report, Developing New Vocational Pathways, will be seized upon by critics who argue vocational GCSEs are soft-options. It is expected to criticise some courses for not stretching the most able.
The report follows inspectors' recent criticism of the pound;40 million increased flexibilty programme which gives 14 to 16-year-olds the chance to study in further education colleges.
It comes as exam bodies privately express concern that schools are offering the exams as a short-cut to improving results and a growing concern about qualifications worth more than one GCSE.
Peter Toft, Ofsted specialist subject adviser told a conference on engineering in schools: "If vocational courses are to be worth two GCSEs then pupils should be spending twice the length of time on them. In a lot of schools the volume of work does not add up to two GCSEs. It is very worrying."
Of the vocational GCSEs on offer, only applied science courses are consistently worth two GCSEs, he said.
Mr Toft said that the teaching of engineering GCSE often fails to stretch higher ability pupils but the standard of work expected is "pretty akin" to other GCSEs.
"There is a tacit assumption that this is for average or less able pupils but GCSEs should cover the full spectrum of ability including the very bright," he said.
Ofsted is concerned that pupils spend too much time writing about engineering rather than doing it.
Take-up of vocational GCSEs has so far exceeded expectations and heads accept that they must not be seen as an easy option.
John Dunford, general secretary of the Secondary Heads Association, said:
"It is vitally important that vocational awards should be equivalent in both standards and workload to their academic equivalents."
Professor Alan Smithers, of Liverpool university, said it would be better to offer slimmed-down vocational GCSEs. "This is a wake-up call. The Government should look again at the content of these courses. Single vocational GCSEs would give young people greater flexibility to try more than one area," he said.
Earlier this year, the Qualifications and Curriculum Authority said it was concerned that schools were choosing some qualifications because they would help their league-table position, rather than on their educational value.
Concern has so far focused largely on the intermediate GNVQ in information and communications technology, which is worth four GCSEs at C grade.
Schools were often spending a "disproportionately" small amount of time on the qualification, the QCA said. Six of the top 10 secondaries in the list of last year's most improved GCSE results admitted that introducing the course, which will be scrapped in 2007, had boosted results.
Last year, The TES revealed how Thomas Telford school in Telford, Shropshire, made pound;7 million in two years selling its online GNVQ courses to schools keen to move up the league tables.
NON-ACADEMIC STEPPING STONE
Vocational GCSEs are the Government's latest attempt to raise the status of non-academic education in schools.
Ministers hope they will help meet the UK's craft and technical skills shortage by acting as a stepping stone for pupils wanting to move on to apprenticeships.
Ministers are examining whether to introduce vocational GCSEs in other subjects. Some of these may be smaller courses equivalent to one ordinary GCSE.
The new qualifications are designed to introduce students to a broad section of industry or business and provide them with hands-on experience.
Grades, from A* to G, are based on exams (30 per cent of total mark) and a portfolio of work produced throughout the course (70 per cent).