The Educational Institute of Scotland (EIS) said the Government plan would also undermine the wider aim of ministers to encourage greater study of Scottish classics.
The attack comes just days after exam officials warned that increasing numbers of pupils were already using prepared answers in their Higher English exams.
In January, Michael Russell, the Education Secretary, announced that candidates sitting Higher English from 2014/15 will answer at least one question on a Scottish novel, play or poem.
The announcement followed a recommendation from the Government's Scottish Studies Working Group that new National Qualifications should include a specific element on Scottish texts in the external assessment.
The move aims to ensure future generations of Scottish young people grow up with an understanding of their culture and literary heritage.
Teachers argue they already use Scottish texts alongside established classics, but need flexibility to allow them to follow the interests of pupils.
The EIS has approved an official paper opposing the mandatory element in examinations.
The paper states: "The introduction of the compulsory exam question on Scottish texts will severely weaken and unbalance the new English courses, severely compromise central tenets of the new curriculum and, ironically, lead to a diminishing of Scottish literature and culture in the eyes of young learners. Breadth, depth, challenge, enjoyment, choice, coherence and relevance – these are principles worth placing at the heart of the assessment regime of a new curriculum that seeks to be radical and are the true progressive means of engendering a life-long love for the literature and culture of our nation.
"Lists of analysis questions out of 20 marks on the same set texts year after year simply are not."
Larry Flanagan, general secretary of the EIS, added: "This will detract from deep learning and critical thinking, and encourage the type of teaching to the test that was such a disliked feature."
The Scottish Secondary Teachers' Association has already attacked the move as unnecessary because most English teachers already taught Scottish texts.
A Scottish Government spokeswoman said: "The Scottish Government has been clear for the past year that strengthening learning about Scotland should be integral to the development of our school pupils and 90% of the public are on board with this approach.
"The Scottish Studies Working Group considered, at some length, various representations on the issues of Scottish studies and concluded that a specific Scottish element – alongside a wide range of other materials – would be the best way to promote Scottish literature in the classroom, and ensure consistency across all Scottish schools."
Last week, The Herald revealed that officials from national exams body the Scottish Qualifications Authority were concerned that pupils were increasingly using prepared answers in their Higher English exams.
The report states: "There was more evidence than in recent years of candidates coming to the exam with prepared answers, often on questions from recent exam papers, and attempting to adapt these to fit the questions asked.
"It was disappointing to see, once again, that in some centres whole classes had been set identical tasks. This is very unlikely to generate work of quality.
"While class exercises are a worthwhile tactic in the teaching and learning process for writing, candidates should be allowed the freedom to choose the nature of their final submissions."
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