Organisations including the Secondary Heads Association (SHA), the National Association of Headteachers (NAHT) and the Association For Colleges (AFC) argue that the diploma, a major proposal in last week’s mammoth 700-page Review of 16-19 Education, is too prescriptive, discriminates against scientists and would only serve an academic elite.

They will be urging the Government to give sixth formers much more choice in the subjects studied for a diploma, allowing specialisation as well as breadth.

Many of Sir Ron’s 200 recommendations received immediate acclaim, including his central proposal for a single qualifications structure. This would allow easy comparison between examination levels in A-level, general national vocational qualifications and the workplace national vocational qualification.

But he also proposed two new over-arching awards: a National Certificate, rewarding students for the quantity of passes in the three types of course; and a National Diploma, gained for passes in four compulsory areas.

The diploma has been the focus of some unease. Sir Ron took the idea from the National Commission for Education. Successful candidates would pass A-levels, AS-levels or their equivalents in: the arts; maths or science; a modern language; and a civics based subject (such as law). They must also have demonstrated ability in the three “key skills” of communication, numeracy and information technology.

The idea has proved attractive to many, but the heads believe it is unworkable.

“It won’t happen,” said John Dunford, president of the SHA. “Virtually no sixth-form or college student is doing that combination at present. Unless there were cast-iron evidence that there were benefits, they’re not going to take it up at the age of 16.”

There are a number of technical difficulties: the fact that sixth formers would find it hard to learn enough science for university courses (if they could not specialise) is just one.

There is also the perception that at present the diploma is only truly available through A-level and AS-level courses. There is little confidence that the NVQ or the GNVQ are comparable with A-level standards.

Moreover, said Mr Dunford, there is no evidence that the universities or employers will reward students for having made the effort to take the diploma.

To be effective, a single qualification would have to be compulsory, rather than optional as is currently proposed. And it would have to be accessible to all students, which would mean merging the three separate routes to qualification.

Many of these points were put to Sir Ron in a formal submission from the Joint Associations Curriculum Group, a body including the SHA, NAHT, AFC, the Head Masters Conference and the Girls’ Schools Association.

David Hart, general secretary of NAHT, is more positive about the diploma than Mr Dunford, but agreed that Sir Ron’s four areas are too restrictive.

“We found the idea that you have to take four areas out of four unduly limiting,” he said. “We would have preferred to see three out of five or even four out of five.” Students, he pointed out, will already be getting breadth through courses in “key skills”.

“What I don’t want to see happen is people giving up the ghost on the diploma. I think we should persuade the Government that Sir Ron has got his four subject areas wrong.”

While agreeing with this last point, Alan Smithers, director of the Centre for Education and Employment Research at Brunel University, believes that such concerns relate to a more basic difficulty. Sir Ron, he said, has the wrong idea of “breadth” in the first place.

“The diploma isn’t really feasible at present because to introduce it you have to describe the breadth and make sure that all students have an opportunity to achieve that breadth. At the moment, this is not possible through the GNVQ or the NVQ.

“Breadth is all right as a general idea. It’s certainly good for people who want it. The difficulty arises when you start to specify what breadth is. I have my doubts about whether it’s desirable.

“Post-16, a lot of people will indeed want breadth and the real weakness of the A-level system was that there was no chance of it. At the same time, many people know exactly what they want and wish to specialise.” If anyone defines breadth, he believes it should be the students through the courses they choose.

Meanwhile, as he noted, the building blocks for an amended National Diploma, or even a baccalaureate, are in place should a future administration desire one.

Whether the employers or university admissions tutors would support a diploma is harder to say. Employers have, somewhat perversely, relied on university definitions of what constitutes a good qualification: after all, the A-level is a university entrance exam. And it is with such definitions that the problem has arisen. Neither group has yet received its copies of Sir Ron’s report.