Ministers' decision to insist that all GCSE exams be taken at the end of courses will increase the risk of repeating this summer's exam board errors, an assessment expert has warned.
This week, education secretary Michael Gove announced an end to modular GCSE assessment that allows pupils to be examined in January and halfway through their courses on "bite-sized" chunks.
But work on new GCSEs purpose built for linear assessment will not start until at least 2013. Professor Alan Smithers is concerned about the impact of interim arrangements on exam boards whose string of mistakes this year led to questions in Parliament.
The Buckingham University academic fears moving January exams to an already congested summer timetable could lead to the boards "making more mistakes".
"This transition will make it very difficult for pupils and exam boards, who will be expected to produce the examinations all at once when they are used to doing it over a longer period," he said.
All pupils starting GCSE courses from September 2012 will be assessed with end-of-course exams. But the GCSEs themselves will not be changed until after some time after a new national curriculum is agreed in 2013.
Professor Smithers agrees with the idea of linear exams but believes it would be better to introduce the changes all in one go.
"Clearly examinations designed as modular over a two-year period are actually going to be rather different when bunched up together at the end of courses," he said. "It may be that pupils in the interim are going to have the worst of all possible worlds."
The OCR exam board, involved in five of the 11 errors or complaints being investigated from this summer, said the costs and logistics of the GCSE changes would need "careful consideration".
Ofqual, the exams regulator, has warned ministers that making the change too quickly could "damage" the quality of teaching and "students' chances in exams".
"Some teachers who are used to teaching modular GCSE courses will need time to revise their teaching plans and strategies," Glenys Stacey, Ofqual chief executive, said in a letter to Mr Gove.
She advised him that bringing in the change this year would involve "unacceptable risks" but they would be "substantially" reduced by the delay until 2012.
The end of modular exams will mean the end of mid-course resits, which Mr Gove said had reduced teaching time with too much exam practice.
"This meant that less time was being spent on developing a deep and rounded knowledge of the subject," he said. "I think the modular system was a mistake, and the culture of re-sits is wrong."
Mark Dawe, OCR chief executive said: "A return to a linear structure will help reduce the dangers of over-assessment of young people, give more time for teaching and increase the opportunities to teach whole subjects in a joined-up way rather than in bite-sized chunks.
"However, the costs and logistics of implementing all these further changes will need careful consideration."
The Department for Education argues the end of resits will reduce pressure on the timetable and that many pupils already sit all their GCSE exams at the end of their course.
A spokesman said: "This is about restoring rigour to our exams system so that it keeps pace with the best around the world."