However, new criteria published by the Qualifications and Curriculum Authority should give teachers a clearer picture of the revised versions of exams, which they will begin teaching from September 2008.

Changes include reducing the number of units in nearly all subjects, except sciences and music, from six to four (see box).

But it is unclear how new higher grades such as a possible A* will work.

The criteria simply state that subjects must “contain sufficient demand ...

to allow recognition of performance above grade A”.

Last year’s Government white paper on 14 to 19 education proposed new sections at the end of A2s which pupils take in their second year of A-levels. These would comprise optional extra questions, similar in difficulty to those in the existing but little used advanced extension award exam.

This approach would allow pupils to receive an A grade, or even a B grade at A-level, with “distinction” or “merit” depending on how they did in the extra section.

However, the QCA wrote to the Government in October suggesting an alternative approach. It proposed mixing harder questions into the A2 exam, then introducing a new grading scale which would retain the existing A to E grades but adding one or two higher grades, opening the door for A*s or even A**s.

Ruth Kelly, the then education secretary, wrote back to the QCA in February telling them that ministers still preferred their original idea but would allow the qualifications watchdog to trial both.

Officials at the QCA are now creating model versions of three approaches, using questions from this year’s papers: the first based on the Government’s proposal; the second on its suggestion; and a third where pupils would sit a separate, harder paper as they do with the advanced extension award.

They expect to test these papers with pupils in June before producing final specifications in September 2007, a year before the exams.

The DfES insists it will keep an open mind. “We will await the trials that the QCA is carrying out and see how we can increase stretch at A-level depending on their outcomes,” said a spokeswoman.

However, the QCA has also promised it will publish details so that exam boards know how to add “stretch and challenge” to their A-levels this December, before the trials.

A QCA spokesman said that boards would get information to begin designing more challenging questions, even if it was undecided where in the exam papers they would appear.

The Government can take comfort from teachers’ response to the consultation. A majority, including 67 per cent of English literature teachers and 81 per cent of modern language teachers, back making the harder questions an optional section at the end of A2 papers. But a larger majority of teachers have also shown support for putting harder questions into a separate exam paper, even though critics fear less ambitious schools might not ask pupils to sit it.

Teaching unions said they were concerned that decisions over how the new higher grades might work were being left too late.

John Dunford, general secretary of the Association of School and College Leaders, said: “It is not a satisfactory situation. Schools need to have sufficient time between when a decision is finalised and when they have to start teaching the courses.”

Geoff Parks, director of admissions for the Cambridge colleges, said universities would be keen that different approaches were given a trial before the final one was chosen.

THE REVISED QCA CRITERIA FOR COURSES:

* All A-levels to be reduced to a total of four units from six, except sciences and music.

* A2 exams to contain sufficiently demanding questions to allow new grades above A.

* Pupils to be assessed on their “holistic understanding” of subjects, to improve their synoptic (subject-wide) knowledge

* Internal assessment only where it is the soundest method of assessing specific skills.

* Clearer specifications to be introduced by exam boards to ensure fairness and reliability of internal assessment.