All three major exam boards are opposing plans to offer new diploma qualifications proposed in the Tomlinson Report.
The grading of the diplomas, which would replace A-levels, GCSEs and vocational exams, will be one of the key issues to be resolved before the final report is sent to ministers in September.
The report from Mike Tomlinson, the former chief inspector, says that grades of pass, merit and distinction would help to ensure the qualifications' credibility. If students simply passed or failed, many employers and universities would put more emphasis on the grades awarded for individual subjects. But exam boards say the diploma grades would be meaningless because GCSEs and A-levels and vocational exams are assessed differently.
The criticism comes as a leading academic warned that the diploma system, as set out in last week's 100-page report, would be "incredibly difficult to manage".
The report suggests a four-level diploma, offered at entry, foundation, intermediate and advanced levels, which would absorb all current academic and vocational courses within 10 years.
However, Ron McLone, director general of assessment at the University of Cambridge Local Examinations Syndicate, parent body of the OCR board, said the proposed grading system would be meaningless and unreliable.
The problems arise as different exams have different assessment systems, said Dr McClone. At A-level and GCSE, candidates can fail particular papers but can still pass if they do better in other parts of the course. But many vocational awards are only given to candidates who pass all the elements of a course.
Dr McLone, who also chairs the exam boards' umbrella group, the Joint Council for Qualifications, said that this meant that it would be impossible to come up with an overall grade which had anything meaningful to say about the student. He said: "If you are trying to say, for example, this person has got an intermediate diploma at a merit level, people are going to say 'what the hell does that mean'?"
"You are trying to add up apples and pears, and probably getting an orange."
The report acknowledges that the arguments for and against grading diplomas are "finely balanced". Independent schools are among those in favour of grading.
Meanwhile, Professor Alison Wolf, of King's College, London, one of the country's leading qualifications experts, said that the diploma system would be the most complicated in the world.
"It would be incredibly difficult to manage," she said.
The problems surround the system's flexibility. Learners could transfer "credits" built up in individual subjects to different types of diplomas, taught in both schools and colleges. This has huge administrative and funding implications, said Professor Wolf.
Analysis 20; Platform 21