Thousands of less able pupils face being let down by the new diplomas because of the Government's push to prove they are as academically rigorous as GCSEs and A-levels, according to school leaders and academics.
They believe that the foundation-level diploma, worth five GCSEs at A* to G grades, needs to be broader and more practical, with a less academic approach to basic maths and English.
Dr Ken Spours, co-director of the Nuffield Review of 14-19 education, said he had seen sample questions for the diploma, which will be offered to pupils from September, and had found them "unrealistic". He believes the problem stems from the fact that the partnerships developing diplomas have been led largely by employers.
"The foundation level is below the level of employability, so it is an afterthought for employers," he said. "But it isn't an afterthought for the people in classrooms who have to teach it."
Janet Kane, assistant head for inclusion and access at The Hayesbrook School in Kent - which has a 30 per cent special needs intake - said diplomas had huge potential to re-engage pupils of lower ability, but that literacy and numeracy courses for teenagers needed to be "extremely creative, tapping into their lifestyles".
"These pupils will vote with their feet, so they must have a foundation diploma that is appropriate," she said.
John Dunford, general secretary of the Association of School and College Leaders (ASCL), echoed these sentiments at the association's annual conference last week. He called for more ideas from practising teachers in the design of the qualifications. "With the greatest of respect to many of the diploma designers, many of them know as much about modern learning styles as I know about producing widgets," he said.
The comments follow the announcement that the Government will introduce an "extended" diploma, worth 4.5 A-levels in Universities and Colleges Admissions Service (Ucas) points, in its latest attempt to prove to parents and universities that the diplomas will be as tough as A-levels.
However, the move has increased the number of possible diploma lines available from 51 to 119 by 2013. Headteachers at the conference said the system was becoming too complex.
The association has called for it to be simplified by introducing an over-arching "umbrella diploma", similar to the Welsh Baccalaureate, which would incorporate many existing qualifications, including GCSEs and A-levels.
Dr Dunford said: "There is huge uncertainty among our members as to whether, even in partnerships, we can deliver 17 diplomas, each at three levels, plus a wide range of GCSEs, perhaps some NVQs and a choice of A-levels.
"The secretary of state has said that it will be for the market to decide whether A-levels survive. But markets, as we know, are callous beasts, favouring the strong and finishing off the weak.
"You can't put the future of children on a high wire and then just hope that some don't fall off."
Concerns were also raised about take-up for the first five work-related diplomas. Heads in Brighton and Hove, Cheshire and Reading have already reported to the ASCL that low numbers are threatening the viability of some courses, although final figures are not yet available.
Richard Wing, deputy head of Blatchington Mill School in Hove, said there had been very little interest in the society, health and development diploma being offered by his local consortium.
"There is a danger at this stage that we won't be able to run it," he said. "But it may become more popular after the GCSE results come in."
Jane Lees, head of Hindley Community High School in Wigan, Lancashire, said: "We have to win hearts and minds, and the Government has not done that yet.
"It's so complicated. As a head, I have found it hard to convey."
But Ken Boston, chief executive of the Qualifications and Curriculum Authority, issued a challenge to those heads who are complaining of the size and complexity of the diploma system.
In a conference debate, he acknowledged the "extraordinary difficulties" of introducing the system and admitted that the project had not been communicated clearly to the public.
But Mr Boston added: "Such challenges are what leadership is all about. I think it's not so complex as it seems."
Once the concept had been "stripped down to the chassis", he said, pupils and teachers would be less daunted.
He did concede, however, that "universal" access to all diplomas in all parts of the country might not be possible.
Slow the diploma juggernaut, Geoff Barton, Page 32.