Professor Alan Smithers, of Buckingham University, has called on ministers to scrap the diploma, which they believe could become the qualification of choice for 14- to 19-year-olds within five years.
Professor Smithers said it was not clear how pupils would benefit from gaining a diploma that would exist alongside GCSEs and A-levels from September and could eventually replace them.
The new courses, he said, had suffered confusion: they were "trying to be all things to all people".
Professor Smithers published a report today on the progress of the diplomas, which he carried out for the Gatsby Foundation charity with Pamela Robinson, a colleague.
"There is no doubt that vocational education is a mess and A-levels are not doing their job properly," says the report.
"But the solution is not to overturn everything. It is to sort out the difficulties by improving A-levels and putting in place good routes from school to work."
The criticism will add to the controversy surrounding the qualification and comes barely six weeks after Jerry Jarvis, managing director of the Edexcel exam board, said the diploma was at risk of being worthless to pupils.
The charity's report says that as the diplomas will be mainly assessed through coursework, it will be difficult to ensure consistency of marking across different schools and colleges, let alone across different subjects.
"The annual cries about A-level standards will be as nothing compared with the uproar the diplomas will provoke," it says.
It also cites problems for a group of London schools that are trialling the engineering diploma. They had been unable to agree common timetables and had put design and technology teachers in charge, when maths and science staff were needed, the report says.
It adds that a key requirement - that the diplomas include 10 days' compulsory work experience - may prove too much for employers.
This may explain why work experience in any industry, and part-time-work, will count towards the qualification, rather than just placements related to the subject.
Professor Smithers is a longstanding critic of the Government's 14-19 reforms, including the Tomlinson report, which first proposed diplomas in 2004.
A Department for Children, Schools and Families' spokesman said: "It's disappointing this report amounts to more carping from Professor Smithers rather than an attempt to engage positively with the diploma - which has received widespread support.
"It offers an exciting and unique mix of practical and theoretical learning that can be personalised to suit individual needs. That's a strength, not a weakness." l The Diploma: A Disaster Waiting to Happen?