Examiners say they detected "blatant copying of material from the internet" in some of this year's coursework for GCSE English.
Examiners wondered how much was students' own work
Staff at the AQA exam board are said to have been surprised at some of the more obvious examples.
They also said some schools gave students so much help it amounted to "a kind of mass plagiarism".
Another board, Edexcel, said teacher guidance stretched or even went beyond what was acceptable.
The Qualifications and Curriculum Authority (QCA) watchdog is shortly to publish the results of a two-year inquiry into cheating in school exams.
Coursework is expected to be one of the main areas identified as a problem.
The AQA examiners' report deals with the version of the board's English syllabus taken by most candidates this June - more than 424,000 in all.
It said there had been considerable publicity about the availability of essays on the net.
Moderators - who check samples of the coursework which has been marked internally by schools - "found some quite serious instances of blatant copying of material".
They warned that candidates found to have cheated would always be reported and in serious cases most probably would be disqualified.
"Moderators have expressed some surprise at the more obvious examples they have seen," the report said.
The style of some written pieces was so different from the rest of the candidates' work they wondered how teachers had failed to challenge it, especially as they knew the candidates much better than the moderator.
In some schools many candidates responded to a set text in such similar ways it was hard to tell them apart.
"Often moderators would find several paragraphs beginning with exactly the same sentences, and paragraphs would be arranged in identical order."
Schools are allowed to use "scaffolding" - giving students an outline of the structure, to marshal their thoughts.
"Scaffolding is undoubtedly an effective strategy for helping students to structure writing," the examiners said.
"But a distinction has to be made between using scaffolding as a teaching tool for individual learners and using it en masse with classes for a single task which is going to be submitted for assessment."
They felt that in many cases it restricted candidates rather than helping them.
"In the most severe cases, moderators found themselves having to make judgements about whether there was so much scaffolding and so little of the candidate's work that it was a kind of mass plagiarism."
'Teaching by numbers'
AQA's staff were not alone in having concerns.
At Edexcel, examiners said there were very few examples of blatant plagiarism, though it was "still very much an issue".
They added: "More insidiously worrying is the growth of what one moderator described as 'teaching by numbers' and there were other references to 'over reliance on teacher notes' and 'similar responses within a centre'.
"In such cases teacher guidance to candidates stretches what is acceptable to the limit (and beyond) by providing over detailed essay plans, which specify what should go in each paragraph, including the points to be made and the quotations to be used," their report said.
Among other things this made external moderation very difficult, if not impossible, "because it is unclear what work is the pupil's own".
Edexcel's design and technology examiners saw more instances this year of candidates writing "unsuitable and inappropriate comments about their teachers and the exam in general".
They added: "This must be discouraged as it can be, and in some cases, it is, very offensive."