But most schools and colleges will welcome the new exams, which are designed to broaden sixth-form studies and bring them closer to a continental-style baccalaureate.
Under the arrangements, to be introduced from September next year, pupils will be able to take up to five subjects in the first year of their sixth- form studies, instead of the present three, and take new "advanced supplementary" exams at the end of the first year.
Ministers hope English candidates will, for example, take maths while others might mix science and languages.
After a year, students will choose whether to continue with four or five subjects, or to concentrate on three for the final A-level examinations.
Heads and university admissions officials predict that AS exams will revolutionise university entrance. They say that universities are likely to make conditional offers of places based on AS results rather than on teachers' predictions of final A-level grades. Some heads forecast that offers will be made solely on the basis of AS grades.
The course will be offered in six modules, with exams that can be taken throughout the two years and mixed and matched with advanced vocational qualifications.
There will be new, voluntary tests in business skills: communication, numeracy and IT, and the number of A-level syllabuses will be reduced drastically.
The proposals, put forward in a report by Lord Dearing, chairman under the last government of the Schools Curriculum and Assessment Authority, come after years of complaints from schools that A-levels are too narrow.
The plan has divided ministers, with the Prime Minister and his advisers determined to avoid the accusation that they are lowering standards. William Hague, Tory leader, has warned that the A-level "gold standard" should not be threatened.
Tessa Blackstone, the higher education minister, originally wanted more fundamental changes, with an 18-plus exam closer to the European model.
While both state and independent school heads are, in principle, in favour of a broader exam, they are worried about practicalities. They believe ministers have underestimated the upheaval which the increased number of options will cause.
John Dunford, general secretary of the Secondary Heads Association, said: "The new exams will enable students who at the moment drop out of A-level to get some accreditation for a worthwhile performance. But we are concerned that there should be enough resources to provide the extra teaching time."
Vivian Anthony, secretary of the Headmasters' and Headmistresses' Conference of leading independent schools, said: "The real concern of some of the most academic schools is whether we are losing rigour by chopping up courses into modules. We are worried that it will be difficult to design syllabuses suitable for both the ablest pupils and those of lower ability who will take AS levels."
Some heads argue that the changes do not go far enough. The National Association of Head Teachers wants a compulsory baccalaureate.Reuse content