Plans to replace A-levels with the new diploma could be a "disaster", two academics have warned.
Professor Alan Smithers and Dr Pamela Robinson of the University of Buckingham have voiced doubts about plans for the new qualification, which is due to be introduced by nearly three-quarters of schools and the vast majority of colleges this September.
They are the government's first step on the road to replacing GCSEs and A-levels with the preferred combination of practical skills and theoretical knowledge.
"There is no doubt that vocational education is a mess and A-levels are not doing their job properly, but the solution is not to overturn everything," the academics said.
"It is to sort out the difficulties by improving A-levels and putting in place good routes from school to work. This would be simpler, sounder and have a much greater chance of success than throwing everything into the melting pot.
"The original plan was to let employers devise 14 vocational diplomas to meet their needs, but this has morphed into a qualification, the diploma, to be all things to all people."
Schools minister Jim Knight rejected their view as "one-sided carping".
Dr Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said: "This report argues against fundamental change.
"However, our current education system is letting many of our young people down resulting in frequent complaints by universities and employers about the lack of skills such as team-working, effective communication and independent research in those emerging from our schools.
"By ignoring the original reasons for change and only focusing on what he believes are the negative aspects of the government response, he fails to offer any real alternative."