Members of the biggest teaching union will vote on a boycott of next year's national tests despite the view of their colleagues in two other classroom unions that action would be illegal.
Delegates at the National Union of Teachers' conference voted unanimously to ballot on a boycott of key stage 1, 2 and 3 tests.
The vote comes as Charles Clarke, the Education Secretary, faces calls to sit a paper when he meets parents opposed to the national tests in his Norwich constituency next month.
Teachers' anger against the pressure put on teachers and pupils by the tests is growing. Doug McAvoy, the union's general secretary, predicted that members would overwhelmingly back the ballot which, he said would effectively scupper tests for seven and 11-year-olds, next year.
Members of the National Association of Schoolmasters Union of Women Teachers meeting in Bournemouth demanded that league tables be abolished and tests reviewed.
But general secretary Eamonn O'Kane told a private meeting at his union's conference in Bournemouth that a boycott could fall foul of employment legislation. The Association of Teachers and Lecturers has also refused to back a boycott.
Lesley Connolly, secretary of the NASUWT Birmingham association and a former primary teacher, said: "A boycott is not what we could legitimately do. We want these tests abolished.
"They are distressing to teachers and to children. The bewildered look in their eyes, the unhappiness and stress even at the ages of six and seven is quite apparent."
A report called Testing to Destruction which describes national tests as "a competitive, threatening, learning experience" was presented to the NASUWT's conference.
It says that "the hijacking of tests for management information has distorted and diminished their value" and adds that the benefit to pupils'
learning is "highly questionable".
The conference heard that a single pupil sits an average of 105 tests between the ages of four and 19. A senior examiner said that she had worked 16-hour days seven days a week from May to July.
Amanda Haehner, chair of the union's education committee and a Croydon English teacher, said: "This idea that the only way teachers know how pupils are working is by testing them is ridiculous. You are in the classroom with them every day for a year, every day you know what level they are at."
A call from teachers from Birmingham and the London boroughs of Lewisham and Tower Hamlets for the abolition of tests at key stage 1 and 3 tests was due to be debated by the NASUWT conference today.
John Illingworth, a past-president and a candidate to succeed Mr McAvoy, told conference delegates he he would challenge the Education Secretary to sit one of his own tests when he holds a debate with him next month.
Parents in Mr Clarke's constituency have organised the debate as part of a campaign against testing in local schools.
Mr Illingworth announced the challenge after hearing that children's author Pat Thomson had failed to gain full marks in a KS2 English paper based on her own story. "If storytellers and headteachers can't do the bloody things, it is not surprising that kids can't," he said.