Plans for teachers to mark pupils' exam work, proposed in the Tomlinson review of secondary education, could lead to a huge increase in bureaucracy, say exam boards.
Under the proposals, in-course assessment, rather than traditional exam-hall tests, would be the main way of judging students' achievements at all levels of a diploma that would replace GCSEs and A-levels.
Exam boards say proper "quality assurance" procedures will be needed to check teachers' marking and reassure the public that pupils' grades are accurate.
In its response to Tomlinson, the exam board OCR said there was much in it to welcome but that the proposals on teacher assessment could add significantly to the costs of the new system - financially and in terms of the burden on staff.
It said: "(Teacher) assessment, if it is to be reliable, requires standardisation and quality assurance mechanisms, which require expertise and resources not currently in place.
"The cost and bureaucracy of devolved assessment can be considerable. It is particularly important that government and the (Qualifications and Curriculum Authority) recognise the potential additional burdens on teachers as well as the benefits."
Mike Cresswell, director general of the AQA board, voiced similar concerns, and said he was worried about a diagram in the report that suggested eight stages in the proposed internal assessment system, from boards designing a syllabus to pupils receiving a grade.
The diagram suggests that schools and colleges would develop their own ways to assess pupils' performance. It also says teacher assessment would be checked by exam boards' moderation of scripts, comparison of results against national figures and by an inspection system.
Dr Cresswell said he was not against teacher assessment, but that it would work only if the Government was prepared to trust the profession, rather than adding extra red tape.
Mike Tomlinson, the former chief inspector of schools who led the Government's 18-month inquiry, said the shift to teacher assessment would only take place if it could be shown not to add to professional workloads.
The Government will announce its response to Tomlinson early in the new year. Teacher unions are also warning that the proposed change could add to their members' workload.
OCR said many ideas in Tomlinson were to be welcomed, but a suggestion that pupils receive grades for their diplomas is "completely unworkable".
The boards believe it would be virtually impossible to give meaningful grades for the whole qualification when pupils could achieve this qualification through widely differing courses.