The launch of the new qualifications, which depend on the support of employers and are designed to transform vocational education for hundreds of thousands of pupils, is being rushed and will allow no time for proper trials, they say.
The reforms were the centrepiece of February's 14-19 white paper, published in response to last year's Tomlinson report. The diplomas are intended to provide high-status work-related courses.
Diplomas in five subjects, including engineering, health and social care and construction, are being introduced by 2008, six years ahead of the timescale recommended to ministers by Sir Mike Tomlinson. The diplomas will not be trialled.
Sir Mike, who led the review, has said he fears the Government will repeat the error of the Curriculum 2000 reforms, which led to the A-level re-grading fiasco of 2002. "It looks as if it may result in there being insufficient time for proper piloting," he said. "I do not want to see the same mistakes made again."
A senior source from one sector skills council, one of the employer-led bodies involved in developing the diplomas, said: "It is going to be really difficult to design a national diploma that is fit for purpose in this timescale."
Teresa Bergin, of the Qualifications and Curriculum Authority, said the diploma schedule was achievable.