Ministers plan to boost GCSE results by delaying two big-entry exams in order to give pupils more time to prepare.
Revision time for less popular subjects, such as economics, music and religious studies, may be cut as part of a drive, initiated by education minister Baroness Blackstone, to improve grades in major subjects.
From the summer of 2001, English literature and geography students will gain at least two extra revision weeks - at the expense of minority subjects.
The move has been criticised by minority subject organisations. They accuse ministers of manipulating the exam timetable without the research evidence to support the decision.
However headteachers' associations welcomed the move which will mean most candidates will no longer sit papers before June. They hope a normal teaching timetable can continue through the first half of the summer term before major exams start.
Arthur de Caux, senior assistant secretary of the National Association of Headteachers, said: "We would prefer all GCSEs to be taken after half term. But if time constraints mean that can't happen then we would prefer that at least the large entry exams are taken after half term to keep disruption to a minimum."
New Labour has set challenging targets for GCSEs. Ministers want half of all 15-year-olds to achieve five good passes by 2002, compared to 46.3 per cent this year.
Results have improved by around one percentage point a year for the past three years but more rapid increases are needed to ensure the target is reached.
English literature and geography are the only two popular subjects currently taken before the summer half term.
Less popular exams are to be moved forward to fill the gap in the timetable because officials want the maximum number of students to benefit from extra teaching and revision time. They have yet to decide which subjects could lose out under the change, the first for five years.
Lat Blaylock of the Professional Council for Religious Education, said: "If ministers didn't believe this change would have a significant impact on performance, they wouldn't be doing it. "RE is the fastest growing GCSE and our candidates deserve the maximum time to prepare for the exam. If the Government decided that RE would lose out we would demand that it be put back."
History is not immediately at risk, but has a small entry.
Ben Walsh, chair of the Historical Association's secondary committee, said:
"We are not convinced that the Government has any scientific rationale for these changes. It may even work against them because of cumulative exam fatigue which means that candidates are worn out by the time they have taken eight or nine papers."
A spokesman for the Qualifications and Curriculum Authority said: "This change will reduce the number of written papers most candidates take before half term. We consulted widely with teachers associations and teachers on our advisory committee and they were strongly in favour of the change."
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