Teachers fear a GCSE exam in computing has been "blatantly compromised" after students found answers to a controlled assessment task in a course textbook.
One teacher said his class had produced almost identical answers to a practical investigation task in this year's OCR computing GCSE.
The students told him the answers were in a 2012 course textbook co-authored by a senior OCR computing examiner.
But OCR says it is confident the task is not compromised.
The teacher, who asked not to be named, said that when he checked the textbook, written by Sean O'Byrne and George Rouse and published jointly by OCR and Hodder Education, he realised an example problem, with the answers, was almost identical to a task in this year's practical investigation controlled assessment.
The controlled assessment is an investigative piece of work for students, taking approximately 20 hours of their time.
Students are expected to work independently according to OCR guidance which states: "It should be remembered that candidates are required to reach their own judgements and conclusions without any guidance or assistance."
The work is graded by teachers who must submit marks for moderation by mid-May.
"It's really irritating to me as a teacher.
"The students have spent two years learning and now I want to see the results of what they have learned. I want to see a fair distribution of marks for them," said the teacher.
"This disadvantages pupils at those schools which can't afford textbooks.
"Also, weaker kids are going to do better than they should have done while the good kids might think this is too obvious and may mess up because they get confused and will do more poorly than they should do.
"They have to show they can plan, understand the technology and tackle practical skills - but now they have found this is published in a textbook they don't have to."
The teacher added that it would be impossible to know which students had worked completely independently and which had copied answers from the textbook.
Reducing the marks available for this task would disadvantage schools which had opted for this task rather than either of the other two options, said the teacher.
On an online forum another called it "a very serious matter".
"I need to know there is a level playing field for my students who have slogged their guts out over the past year to do each of the 20-hour controlled assessment tasks without any direction from me as dictated by the exam board."
One of the teachers also questioned the advisability of senior examiners writing textbooks in the subject they are examining.
Co-author George Rouse gave a talk to the BETT education technology fair in 2013 as OCR's chief examiner for computing GCSE.
In a statement, OCR said it took the standard of assessments very seriously.
"After investigation, we are confident that this controlled assessment task is not compromised.
"OCR would like to reassure schools and students that candidates will not gain an advantage from copying from a textbook as part of an assessment task."
A spokesman later clarified that students would need to do more than just copy from the textbook to gain good marks - as the task depended on understanding and tackling practical problems.
The exams regulator Ofqual said OCR had notified them of the problem and was investigating.
In July 2014, two controlled assessment units in OCR's computing GCSE had to be replaced after the answers appeared online. OCR says there are no similarities to the most recent incident.