Ministers' plan to bolster A levels by giving elite universities the power to set them could create a two-tier system that damages the credibility of the qualification as a whole, critics are warning.
Michael Gove this week asked qualifications watchdog Ofqual to establish new arrangements that would see research-intensive Russell Group universities, entry to which is usually the most competitive, working with exam boards on both the initial design and the "ongoing development" of A levels.
The detail of the education secretary's letter has led academics and school leaders to argue that the change will widen the gap between subjects already perceived as "hard" and "soft".
Mr Gove wrote: "I would not wish standards in any particular subject to be constrained artificially as a result of any concept of comparability between subjects."
He also wants priority in the A-level reforms, which could come in from 2014, to be given to subjects crucial to Russell Group entry. Critics are concerned that this will lead to a twin-track approach, with subjects such as media studies, which elite universities are less interested in, being neglected.
The Russell Group represents Oxford, Cambridge and 18 other universities including Manchester and Bristol. It will expand to represent 24 universities in August.
Brian Lightman, general secretary of the Association of School and College Leaders, said: "We cannot create a two-tier system. It is absolutely essential that A level is looked at in the round. A level has to be a standard."
John Bangs, a senior research associate at the University of Cambridge, said: "Having different standards in different subjects would be damaging to A levels because a message goes out that there are two forms of A level, with one produced by the Russell Group and a second-tier qualification for those considered to be of a lower grade.
"All those emerging subjects that maybe the Russell Group hasn't detected or would look down on, the message would be that they weren't worth as much."
But Professor Alan Smithers from the University of Buckingham, which is not part of the Russell Group, said a two-tier system was not a serious issue.
"The gulf between the A-level subjects will reflect their potential value in terms of going to university and wider society," Professor Smithers said. "One of the problems is they have been scored the same in league tables and Ucas points, which is not a very helpful illusion."
The academic, who used to help the old Joint Matriculation Board to set A levels when he worked at the University of Manchester, warned that the change, which he welcomed, would become a "mega exercise" if universities were expected to be involved in every A-level subject.
Mr Gove has not specified how universities should be engaged, only that exam boards should have to prove that the institutions have been and that both government and the regulator should "step back" from A levels.
His letter acknowledges that "different universities will also have different requirements", and it encourages "diversity". It raises the potential of the boards working independently, consulting different universities and developing very different A levels in the same subjects.
But Andrew Hall, chief executive of AQA, the biggest A-level board, told TES there would be a "commonality" between universities' A-level needs with differences "around the edge".
He said his board would continue to consult with universities from all groupings.
TES understands that the 1994 Group - another cluster of research-intensive universities including St Andrews, Sussex and Bath - was not warned about the proposals.
Mark Fuller, director of communications at the group, condemned the government for "arbitrarily" selecting the Russell Group.
Professor Robert Coe, an exams standards expert from Durham University, welcomed the principle of removing politicians' influence over A levels, but questioned whether universities would have the capacity to play a major role.
He also warned: "It worries me that if you let the Russell Group loose on A levels, and they understand their brief is to make them more challenging, then that might not be fit for the majority of people taking the exam.
"We moved away from that approach 30 years ago because if you were at the bottom it was just too hard and it wasn't a great educational experience."
Professor Coe said the "least worst option" might be to have a clear two-tier system of hard and easier A levels, so long as pupils understood what universities would accept.
University academics want A levels to include more advanced content for more able students, a Cambridge Assessment study published this week has found.
They also want them to cover core subject areas in greater depth and to encourage critical thinking, independent study, experimentation, exploration and more extensive reading.
Universities want less "teaching to the test", according to the research, and reformed A levels to be less predictable and have more essay and open-ended style questions, with a limit to the number of resits.