Universities question whether new qualification will prepare students for degree courses.
Universities have voiced concern whether 14-19 diplomas will give pupils the knowledge and skills needed by undergraduates.
Vice-chancellors and admissions tutors are worried that the curriculum for the new qualifications, to be introduced in September, will not prepare students for certain degree courses.
They are also concerned that extended A-level projects could lead to increased plagiarism and that A* grades will skew admissions in favour of middle-class pupils.
The findings come from the 1994 Group of 19 research-intensive universities, including Durham, Exeter and Goldsmiths in London.
A report by the group said that much more work was needed on reforms to the 14 to 19 curriculum if diplomas were to meet the high expectations placed on them by the Government.
Despite reservations, most of the universities praised the potential of diplomas. In a cautious welcome, just under half of admissions tutors in relevant subjects said it was "very likely" they would accept diplomas for courses from 2010. Another 14 per cent thought it would be quite likely or possible.
The first five diplomas - in engineering, construction, information technology, creative and media, and health - will be on offer for up to 40,000 14 to 19-year-olds this year.
Detailed course content was only published in September last year, and Ucas, the university admissions service, did not announce until December that advanced level diplomas would be worth 3.5 A-levels.
Steve Smith, vice-chancellor of Exeter University and chair of the 1994 Group, said that without those details it had been impossible for universities to give teachers the information they needed.
The report questioned whether diplomas would develop the rigorous analytical skills needed by undergraduates. There was particular concern that the maths element in the engineering degree was not strong enough.
When the 1994 Group universities were asked if the diplomas would prepare students well for existing courses, only three said yes to the construction diploma. However, several other leading universities, including Cambridge, have previously backed the diploma in engineering as good preparation for an engineering degree.
Changes to A-levels due this year were less contentious. The extended project was generally welcomed, although concerns were raised about the reliability of assessment. The introduction of A* grades will make it easier to discriminate between the brightest pupils, the report said.
But there were worries that the qualification would make universities more socially exclusive.
The group's criticisms are likely to concern ministers, who have said that diplomas could replace established courses as the "qualification of choice" for pupils.
However, Jim Knight, schools minister, said he welcomed the group's findings.
"Majority support from this important group of universities is further evidence that support for the diplomas is building in higher education," he said.
SUPPORT IS 'GROWING'
Pupils at Sir Bernard Lovell School in Bristol will be offered all five of the diplomas being launched this year.
David Turrell, the headteacher, said there had been a strong response from pupils for the level two courses, equivalent to seven GCSEs.
"For advanced level diplomas it is still very early days. It will take time for interest to build and will depend in part on the response from universities," he said.
"We are delighted that several leading universities have given a positive response to the engineering diploma. Pupils can take a maths A-level at the same time to prepare them for engineering degrees, which will be a good fit."
Mr Turrell said universities were "understandably cautious" about any new qualifications, but that support for the dipolmas was growing in higher education.
"Universities will want to look at diplomas over time," he said.
"We are delighted that the new courses were announced in languages, science and humanities.
"It is important that they are accepted by all universities to help them succeed."