The Diplomas combine theoretical and practical learning
Plans for an all-embracing system of Diplomas could spell disaster for England's education system, a report by a leading academic has warned.
Professor Alan Smithers said the new Diplomas could undermine educational standards and fail to prepare teenagers for university or work.
Diplomas, which combine academic and practical learning, will be rolled out in England's schools from September.
Ministers said they had won support from employers, schools and experts.
The first five Diplomas in media, health, construction, IT and engineering will be taught to 20,000 teenagers from the next academic year. Another 12 Diplomas will follow.
England's Children's Secretary, Ed Balls, has said they could replace A-levels as the qualification of choice.
The University of Buckingham academic argued in his report: "The Diploma - A disaster waiting to happen", that diplomas would not improve existing qualifications and had difficulties of their own.
The main problem was that their purpose was "confused".
"It is trying to be all things to all people," he said.
It is trying to create "parity of esteem for vocational and academic qualifications, to provide better routes to school, increase the numbers going to university and improve A-levels", he added
This also meant it was "not clear what the Diploma is supposed to qualify for".
"It is extremely doubtful whether the same qualification can be fit for the wide range of purposes that need to be served such as university entrance and recruitment to different levels in employment," he said.
He added: "In going for an all-embracing framework it seems that there is a serious risk of necessary specificity being lost."
Prof Smithers also sounded a warning over the fact that Diplomas are to be mainly assessed internally, rather than marked externally as A-levels are.
"Internal assessment is likely to mean that Diplomas are of a very variable standard, not only between subjects but also within them," he said.
"With A-levels, strenuous efforts are made to ensure both comparability between qualifications and consistency over time, but even they are not perfect."
And the report suggested pupils will be able to gain the Diploma without having to do any work experience in the field they are studying.
For example, someone on a construction and built environment course may not set foot on a construcion site.
Schools Minister Jim Knight rejected the study, which he described as "one-sided".
"The Diploma has garnered widespread backing from employers, universities, schools, colleges and education experts," he said.
"Its popularity continues to grow, with over 100 universities now endorsing it, a huge range of employers involved in delivering it, and over three quarters of schools and colleges now accepted to teach it.
"Learners and parents are beginning to see that support and that Diplomas offer a good stepping stone to employment, either directly or via university or apprenticeship."
He stressed that the advanced Diploma for sixth-formers would be worth the equivalent of three and a half A-levels for teenagers applying to university.
He also said there would be a balance between controlled internal and external assessment, similar to GCSEs.
John Dunford, general secretary of the Association of School and College Leaders, told the BBC's Today programme that the addition of three new Diplomas would strengthen the reputation of the overall qualification.
"What is certain is that by having Diplomas in science, languages and humanities, which are key areas of study for young people, that the Diploma system is being broadened into these important areas," he said.
"What that does, is it reduces the chance of Diplomas becoming a second-class qualification, in the way that so many vocational qualifications have become in this country, regrettably, over the years."
The shadow secretary for children, schools and families, Michael Gove, accused Ed Balls of "playing politics with the diploma, trying to use it as a way of undermining the A-level".
"His failure to deliver has meant the number taking his new exam has halved, with schools, universities and employers all concerned that once again vocational education has been neglected," said Mr Gove.
Liberal Democrat schools spokesman David Laws said: "Ministers are creating a complex qualifications maze which is likely to be a massive failure.
"The government must change this flawed diploma model completely, or it will be forced to accept the biggest qualifications flop in recent history."
NUT acting general secretary Christine Blower said Prof Smithers had missed the point in his report.
"Teachers support diplomas but they are deeply concerned about the operational implications of their introduction.
"The first year of the diplomas should be seen as a pilot in which lessons are learnt, not as a fixed roll-out."