The Qualifications and Curriculum Authority said that feedback this year for key stage 3 suggested that drilling of youngsters was becoming counterproductive.
The QCA is particularly concerned about schools' teaching of writing "triplets". Part of the national curriculum is devoted to the four triplets, which say that writing should be used to inform, explain and describe; persuade, argue and advise; analyse, review and comment; or imagine, explore and entertain.
English experts say schools encourage pupils to write in an atomised style which accumulates marks by reproducing each of the characteristics in the triplets. The QCA has backed some teachers' warnings that this is ineffective.
A spokesman said: "QCA's view is that the best preparation for the tests is for pupils to follow a rich and varied curriculum.
"Conversations with KS3 teachers... revealed concerns that intensive test preparation for writing, based mainly on reproducing the characteristics of the different writing triplets, was not effective. Some pupils found it difficult to respond to tasks in the test as they were unused to producing work independently without explicit modelling or guidance." The comments to The TES come after Paul Wright, a QCA English consultant, explained his concerns to a meeting on the future of the subject. He spoke of a series of meetings with heads of English about this year's tests.
He said the teachers admitted that they had little time for curriculum enrichment work because of the focus on test preparation. "I asked: 'Are the children able to write?'," Mr Wright told delegates at the National Foundation for Educational Research conference in Slough.
"They said no, because they are so used to being prepared in such a narrow way that when they are presented with a writing test that makes them use their imagination, and calls on them to make decisions, they find it difficult.
"Teachers' preoccupation is to prepare pupils in a narrow way, but this is harming the pupils' ability to write independently."
The TES reported two weeks ago how some schools are spending up to a year preparing pupils for KS3 tests.
This phenomenon is well-known in primaries. The regulator says it has been warning schools for at least three years about over-preparation. But schools say the pressure on them to improve test results leaves them with little choice.