Diplomas are in danger of becoming "second rate" and "second tier" because pupils are being prevented from taking A-levels as part of the new qualification, officials have warned ministers.
An agency formed to promote vocational qualifications wants an "urgent nationwide review" because a "significant" number of Diploma students will be unable to apply to top universities.
The call comes in a letter from London's Linking Lifelong Learning Network, seen by The TES, sent to Ed Balls, Schools Secretary, in July, which warns that the matter is "very pressing".
But the publicly funded agency has yet to receive a reply and was told to resend the letter when it followed up the matter earlier this month.
The news comes in the week that the Advertising Standards Authority (ASA) upheld complaints against Government advertising claims that Diplomas "can get you into any university".
The authority found they breached rules on truthfulness and substantiation, noting that Cambridge University only accepted the engineering Diploma and that a small number of others would not accept all five Diplomas.
Sue Betts, Linking London Lifelong Learning Network director, fears the university acceptance problem will get worse because some pupils are not being allowed to study an A-level as part of the additional and specialist learning (ASL) section of the Diploma - an entry requirement for many universities.
In her letter to Mr Balls she says this is the case in three London boroughs and envisages that the situation is replicated nationally.
"Many HEIs (higher education institutions) are specifying an A-level as an entry requirement; naturally, these tend to be the top-performing HEIs," she writes.
"The danger, therefore, is that a large proportion of advanced Diploma students are debarred from applying to a large number of universities.
"This, of course, will have implications for the qualification as it may, by default, come to be seen as a route to some universities but not all, and become a 'second tier' qualification."
Iain Wright, 14-19 education minister, said: "No student should be denied the option of going to university because they cannot sit A-levels alongside Diplomas.
"The full range of ASL in some areas is still being developed. But we are working with schools to offer students the widest possible choices."
An Association of Colleges survey this week suggested that the Diploma was bogged down by bureaucracy, that some courses were only suitable for bright students, and that functional courses in English and maths were barriers to students gaining Diploma passes.
But the survey of 133 colleges found that 83 per cent of staff teaching the Diploma were positive about the courses and said students enjoyed them.
A DCSF spokesperson said it disagreed with the ASA ruling. The Conservatives said it exposed the advertisements as "dishonest" and the Liberal Democrats said it showed that Government spin was "out of control".
FE Focus, page 3.