The new 14-19 diplomas are a "disaster waiting to happen", a paper from the Centre for Education and Employment Research has argued.
The paper, by Alan Smithers and Pamela Robinson of the centre, which is based at the University of Buckingham, says that the basic concept of the diploma, which may eventually replace the A level, is flawed.
The qualification is trying to be "all things to all people" in seeking to provide better routes from school to work, to increase higher education participation and to improve on A levels by stretching the most able students, the authors say.
"It is extremely doubtful whether the same qualification can be fit for a wide range of purposes such as university entrance and recruitment to craft and technician-level positions in employment," the pair conclude.
They add: "The diploma is to be mainly internally assessed so it will be difficult to ensure comparability of standards in the same subject let alone between them. Access to universities and employment will inevitably be less fair. The annual cries about A-level standards will be as nothing compared to the uproar the diploma will provoke."
They also said that "generic components" of the diplomas, which will include work experience and personal, learning and thinking skills, were of "questionable value".
Deian Hopkin, vice-chancellor of London South Bank University and a government "diploma champion", said: "The principle of a diploma encompassing both academic and vocational education has been welcomed by universities and employers alike. At least 100 universities have a statement accepting diplomas on their websites, including research-intensive institutions.
"Core and functional skills were included precisely to meet the unease many employers felt about existing qualifications.
"Rather than simply castigating the diplomas, we should work together to solve practical issues in the interest of thousands of young people for whom this is a vital new qualification."