GCSE candidates have "cheated" the system by taking crib sheets into their exams, an analysis of last year's papers has revealed.
In response an exam board has banned all pupils from taking work into its design and technology exams after some candidates were found to have abused the system of "open" exams.
Nearly 3,000 candidates will be affected by the Oxford and Cambridge board's clamp-down, taken after a spot-check revealed that some of last year's candidates had flouted the rules.
In previous years students researched one specific topic before the exam and condensed their findings on to an A3 sheet of paper to be taken into the exam.
But a random sample of the sheets handed in with last year's exam papers revealed that some candidates had tried to boost their marks by putting down essential facts about the whole course.
A board spokesman said the change would make the exam fairer for all candidates. He said: "It was getting increasingly difficult to enforce the rules about what could be taken in and what couldn't. An analysis of last year's sheets showed that some people had gone well beyond what was allowed.
"We could not tolerate a situation where candidates who abided by the rules might be at a disadvantage.
"The syllabus and standard of assessment will not be affected, this is just a small variation ."
But schools, which were told of the change last month, are angry that they were given so little notice of what many regard as a significant change.
Martin Allen, head of technology at Swanlea school in Whitechapel, east London, said: "It is outrageous to inform us so late, our students obviously believed they could take the sheet in with them, as they did for their mock and many of their friends did last year.
"We have worked really hard with our students getting them prepared for their exams, with the knowledge that they could take an A3 sheet in with them."