Compulsory national tests for schoolchildren are an unnecessary and stressful burden, according to a survey of teachers in England and Wales.
Teachers would prefer to assess pupils themselves
Only 6% thought the tests a reliable way of evaluating pupils' achievements.
More than eight in 10 of the 30,500 teachers surveyed for the NUT union felt tests were stressful to children.
There was strong support for a boycott of the tests - but the government has said that would be an "absolute
betrayal" of pupils and parents.
The NUT's general secretary, Doug McAvoy, said: "This survey underlines the strong criticisms teachers have of these tests.
"They narrow education, limit use of professional judgement, place unnecessary stress on pupils and add significantly to the workload of teachers without producing any benefits."
The tests regulator, the Qualifications and Curriculum Authority, said its annual evaluation, published on Thursday, "shows that the majority of teachers surveyed feel they are appropriate for assessing pupil performance, and are motivating and engaging for pupils."
On the contrary, said Mr McAvoy: "The government would be hard put to find a teacher who thinks they are beneficial, improve achievement, or promote a broad and balanced education for our children.
"Throughout the report, teachers emphasise the waste of time and energy these tests represent for no real return."
He said the government should let teachers use their own assessments to determine pupils' needs and inform parents of how their children were coming along.
'No going back'
But the Education Secretary, Charles Clarke, said tests were a fundamental part of raising standards in schools.
"They help teachers see what progress their pupils are making. They can also measure progress against other children.
"The national tests are particularly important for children in the most disadvantaged areas. Testing tackles the low expectation culture that used to hold back those children."
Parents valued the feedback.
"Many parents will be alarmed at the threat of preventing them knowing how their children are doing at school.
"It would be an absolute betrayal for teachers to boycott these tests."
He hoped teachers would vote against such action - "strike action", as he called it, though the union said there was no question of that.
A spokesperson for the NUT said: "Parents will continue to receive information on pupil progress.
"Teacher assessment, which gains much support in our survey, will be the basis for all pupils in Scotland and is the basis for pupils at Key Stage 1 in Wales."
The survey was organised for the union by Sean Neill of the Institute of Education at Warwick University.
He found that teacher assessment was seen as a viable alternative to tests by 85% of the respondents.
Dr Neill commented on a recent international evaluation of testing.
"Though the effect of continued testing is to raise test standards, some of this effect can be attributed to increasing familiarity with the test methods by both teachers and learners, increasing emphasis on preparation for the tests, and instruction specifically focused on the predicted outcomes of the tests."
Some anonymous comments from teachers were included in his report.
"Raising standards can effectively be done in schools with good monitoring and self-evaluation practices," said a primary school leader.
Another, teaching infants, said tests "rule" all teaching in a school where children come from poor backgrounds.
"They need enrichment far more than brighter children from more educated families, yet they get less. Save money from Sats and give us support and resources to improve standards."
Almost everyone - 91% - said the tests placed an additional workload on teachers.
A similar proportion of primary teachers, and 85% of secondary teachers, said they were stressful for pupils.
Some felt they managed to insulate the children from this. But others said parents bought revision aids and private tutoring and offered children "bribes" to do well - even in the youngest age group.
Some 90% of teachers felt the tests diminished pupils' access to a broad and balanced curriculum.
The survey showed substantial support for a ballot by the NUT to boycott the tests.
Support was strongest (82.5%) for a ballot to boycott Key Stage 1 tests - the youngest children. In Wales those tests have been dropped.
The support was 71.4% at Key Stage 2, when the tests form the basis of the primary school performance tables. In secondary schools support was 64% in favour of a Key Stage 3 boycott.
The majority of respondents - 67.9% - had more than 11 years' experience in teaching.
Most were in England; 4% in Wales. About 57% were in primary schools.
How teachers responded to statements about the tests: