A-level geography students whose papers are marked online have been advised to keep their answers to length.
Students are being told to write concisely
Sceptics fear the move is technology driven and may not be a fair way of testing students' ability.
The exam board Edexcel denies the approach is to suit the demands of its online marking system.
It says it is linked to "exam logic" - writing concisely to avoid spending too much time on any one answer.
In recommendations which specifically target online marking, students are advised "as a general guide, to write two lines per mark with average sized hand writing and always strive to write concisely".
The advice is outlined in the annual report by Edexcel's chief examiner for A-level geography, assessing this year's exam papers.
Students are also advised to write in blue or black ink and "to use additional writing sheets only where absolutely necessary".
An Edexcel spokeswoman said: "The examiner who wrote this was concerned that students were spending too long on shorter answers and not maximising their marking potential."
But Hilary Sargeant, secondary consultant at the National Association of Head Teachers (NAHT) said: "This is a clear example of the tail wagging the dog."
Back to basics
She added: "We must avoid moving into areas where we set up tests which are easy for markers rather than testing the ability of students.
"We need to go back to basics and not twist modern technology to undertake tasks where it is not the best way of assessing the youngsters' capabilities.
"Online marking has its place. It is essential and will play an important part, but we do have to keep the basics in mind.
"Assessment is a highly developed process and there is no excuse for an easy way out."
In this year's exams any extra sheets used by students were posted to examiners for marking, but from next year they will be available to mark online.
Edexcel insists the online marking does not hinder students.
The exam board's spokeswoman added: "We haven't changed our papers in any way, just digitised them. We set exams in exactly the same way."
Edexcel introduced online marking in 2003 and says 90% of GCSE and A-levels are now marked in this way.
The Qualifications and Curriculum Authority (QCA) believes online marking is the way forward.
A spokeswoman said: "QCA supports the development of on screen marking, in line with the code of practice and our regulatory principles for e-assessment.
"It is important that we have an examinations system fit for the 21st century."